Why don’t more surgeons use obsidian scalpels?

Too fragile
Too expensive
A steel scalpel is every bit sharp enough

… and the main reason is (3).
Interestingly enough, most surgery does not rely a whole lot on the scalpel. While the average person may imagine surgeons in the operating room just going to town with scalpels the whole time, that’s not actually how it goes :-). In most types of surgery, the scalpel is used for making the skin incision, and that’s pretty much “it”.
A super-sharp cutting instrument like a scalpel is good at one thing – cutting – which has both an upside of utility and a downside of potential harms. When you really need to cut, you need a good cutting tool, but a sharp cutter is also indiscriminate… for instance, it goes through blood vessels, and not just the areterial “bleeders” that you can suture off or cauterize, but also through the microvasculature that invests all tissues. So if surgeons relied on sharp dissection to separate tissues, there’d be a lot more bleeding, which would be directly harmful to the patient in the case of significant blood loss, but also an immediate disadvantage because blood obscures the surgical field. The bloodless field is one of the great advances of modern surgery. To achieve this, you need ways of cutting that produce concomitant coagulation, and there are two primary kinds of instruments for this: electrocautery, and ultrasonic ablation/coagulation instruments. Electrocautery includes both “unipolar” and “bipolar” variants. The “unipolar” kind is basically your classic Bovie electrocautery instrument ( so classic, that it’s called a “Bovie” whether or not it’s actually manufactured by the Bovie Medical corporation, which also makes bipolar instruments and other stuff). In a typical open general surgery procedure, the scalpel is used just to make a skin-deep incision exposing the subcutaneous fat. You then set the scalpel aside and take up the Bovie, using it to basically sizzle your way through those fat layers, by distracting the tissue to the sides and then making a stroke with the instrument, and then repeating this process. When you get a bit of bleeding, you can see those focal bleeding vessels and cauterize them directly with the Bovie tip, or you can grab a little arterial bleeder with forceps or a hemostat at the bleeding site or proximal to it, and then touch the Bovie tip to the metal instrument and actuate it, and sizzle off the bleeder that way. Going this way, you very quickly get through the fat layer and expose the peritoneum (membrane layers lining the abdominal cavity) with no significant bleeding, and then you use the Bovie to open up the peritoneum as well.
There are also laser scalpels used in more specialized applications, which have the same sort of advantages as electrocautery and ultrasound instruments, viz. simultaneous tissue ablation and cauterization.
Other specific cutting applications are served by even more specialized cutting instruments. One example of this is the various GI staplers used in bowel surgeries, which simultaneously cut and staple all in one go.
Even more basic than cutting, though, are various forms of blunt dissection . This is where you use a blunt instrument, or often just your fingers/hands, to separate tissue planes and slide structures apart. There’s a very useful technique where you use scissors to bluntly dissect… rather than the way you’re used to using scissors, e.g. with paper, i.e. advancing the open scissors and then closing them to cut between the sharp scissor blades, instead you advance the closed blunt-tip scissor to push your way in, and then spread the blades open to separate the tissues. Anyway, blunt dissection is less traumatic to tissues than sharp dissection and therefore results in quicker and better healing. Also, it runs much less of a risk of causing injury through inadvertent damage to structures, which gets back to your original question and why a sharper cutting instrument is better, generally speaking, but only up to a point . (No pun intended). The sharper the cutting instrument, the less “haptic” or tactile feedback you get when cutting through various tissues, and therefore the more likely you are to carry your cut too far into some structure that you didn’t mean to cut, or to unnecessarily cut through neurovascular bundles (structures composed of an artery, vein and nerve branch running together).
Anyway the point of all this is while sometimes you do need to cut, sharp cutting actually has a limited role in most surgery, and the point of that is that “good enough” cutting really means “good enough” for those few situations in which only sharp cutting will do (like making skin incisions). So, imagine if you had something like a lightsaber, that would just cut through tissue effortlessly… you can see now how that still might not actually be a great thing!
Okay, all that was a hopefully more interesting than boring aside about surgical techniques and the importance of scalpels relative to other ways of opening/separating tissues. There’s also a bigger picture perspective on your question which I think is more interesting…
In the case of the steel scalpel, you have (a) an illustration of the principle that “good enough is good enough”, and (b) an instance of having achieved “good enough”. For another example: adding more pixel density to a display device results in greater image resolution… up to a point , but beyond that it only increases the cost and complexity of the device.
In the case of the scalpel, it’s a bit more nuanced. The only purpose of the scalpel is to cut. All of its utility comes owes to its ability to cut. However, its potential harms also owe to its ability to cut. Of its various attributes, arguably only one of them – sharpness – relates to its cutting ability. So, the sharper it is, the more useful it is (at least up to a point where it no longer makes any difference, sort of like the pixels example), but also the more potentially harmful it is. On top of that, in the case of the obsidian scalpel, you have the other trade-offs relating to brittleness and manufacturing cost. So while an obsidian scalpel is indeed sharper, for most applications it is in virtually every way less desirable than a steel scalpel, and most surgeons would not trade a steel scalpel for a drawer full of obsidian ones.
For another example, consider a riding lawn mower. It’s gotta have some speed to it, because that’s the whole point of the thing, as long as it’s cutting the grass! But a top speed of 50 MPH clearly would be excessive… at that speed you aren’t even cutting the grass, and meanwhile the machine becomes a safety hazard. So more of of something, even an essential quality, isn’t always better.
I would suggest that some variation on these themes will show up underlying the answers to most “why don’t” sort of questions… the themes of (a) trade-offs, in terms of cost/benefit, risk/reward, benefit/harm or whatever the case may be, and (b) good enough is good enough. As my Dad, a retired civil engineer, likes to say: “Everything is an optimization problem.” These are cross-cutting themes (again no pun intended) to be found at multiple levels in technology, health care, economics, public policy, building, manufacturing, international relations, etc.
The opportunities are found in figuring out ways to re-balance or re-align the cost-benefit tradeoffs, or to recognize when current solutions actually overshoot a true “good enough” point which could be achieved with something cheaper/simpler/safer, or by changing the game so that what until now was not good enough is now good enough.

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