foo.h defines i64 as “long” and bar.h defines it as “long long”. Now what?

This would break legacy code. I had one employer with a bool type that was a 32-bit integer because of obscure alignment issues. This is why _Bool exists and the bool typedef has to be brought in explicitly with stdbool. You can't just go and usurp popular short type names. The standard reserved *_t and _[A-Z].+ for this purpose. People have been writing portable code with the expectation that future standards aren't going violate that promise and break things.

This is just one of the reasons that new languages that dispense with outdated assumptions and insted make assumptions more suitable to our times are a good thing. C and C++ are really old and some things can not be fixed with libraries.