Nothing, but what stops you from using DEFLATE64?

Installing new software has a real time and hassle cost, and how much time are you actually saving over the long run? It depends on your usage patterns.

Supporting old APIs and additional legacy ways of doing things has a real cost in maintenance.

So does not supporting them, but the developer gets to externalize those.

the developer is hired by someone that gets to make that decision. Ultimately the customer does. Thats why some people spend extreme resources on legacy crap, because someone has deemed it worth it.

what stops you from installing win95 and winzip?