Actually you could. They are called "deterministic builds".

But this would require some kind of effort from project maintainers.

I'm quite unknowledgeable about this, but from what little I understand: That is actually a very major effort in many cases, isn't it?

You cannot do that for iOS binaries, because they are signed by Apple before publishing on the Store (and so the hash will change).

You can avoid the signature when hashing.