Why is old cast iron better than new cast iron?

It is not necessarily better if you are primarily referring to cookware. Cast iron is what early producers were first able to obtain when they were able to achieve furnace temperatures high enough to separate iron metal from its ore in molten form that could be cast into impure but mostly (typically 90%+) iron products.
Today, in most industrialized parts of the world, iron ore is converted directly into steel via a continuous production process verses being cast into ‘Pig Iron’ ingots which typically contain 4% to 5% carbon, significant silicon, and numerous other elements like sulfur, phosphorous, etc. that were in the ore, and/or the fuel used to smelt it.
Pig iron was typically reheated and/or remelted for making other products like steel or cast iron items.
One should read this Wiki article to understand how cast iron differs from cast steel: Cast iron – Wikipedia
In preparing to write this answer, I watched the short foundry video of a leading USA maker of ‘Cast Iron’ cooking tools. I observed what appeared to be large quantities of various scrap steel items being loaded into its melting furnace and became suspicious that its current products might be nothing more than cast steel; which is likely true of some so called modern cast iron items imported from Asian lands.
After finding and watching the USA maker’s ‘long version’ foundry tour video, I concluded that it is making its product from a well controlled iron alloy that qualifies as real ‘Cast Iron’. The cast iron that it makes today is likely much more uniform and has higher freedom from potentially toxic elements that most cast iron cookware typically made before WW II.
There are potentially significant health problems associated with antique cast iron cookware found in places like flea markets in that one can never know what has been cooked or melted in them in the past. How would you like to cook your bacon and eggs in an old frying pan that had been used to melt scrap lead containing significant arsenic or had had mercury poured into it? While the chances of having any of the foregoing be the case are relatively slim, such are not unknown; and I personally know of several such examples over the past 60+ years!!!
If you are going to cook with old cast iron cookware of unknown origin, you would be wise to thoroughly clean it by washing with a caustic solution, then abrasive blasting it to bare metal, then washing again, and then having it checked with a XRF gun for toxic elements like lead and arsenic and mercury.

Calphalon 10-Piece Pots and Pans Set, Nonstick Kitchen Cookware