Because we want people to write simple idiomatic code that runs fast today and also in ten years on alien hardware.

> Maybe I'm doing some rounding?

Compilers won't do this optimization when it is illegal to. If you disagree with the compiler's idea of what is legal, you can either write inline assembly, or put this code into an always inlined, but never optimized function.

Because we want compilers to perform the optimizations, not programmers.