Primarily because it would make it more difficult to detect and debug out-of-bounds reads, so at the very least it's not something you'd want to do by default, even at high optimization levels.

Possibly it might also be an awkward, complex, or dangerous (as in risk of unintended consequences) optimization to selectively violate the memory model that way. But I'm not familiar with the internals of optimizing compilers, so that's just conjecture.