The problem is that C is sufficiently primitive that optimizing it is effectively trying infer structure backwards because the language doesn't specify it.

For-loops are a great example. Why are we even discussing about a "loop index"? Because we need to iterate over an array, string, etc. and "length" isn't something stored with the data structure so the compiler can't do the loop for you.

The real question is probably more along the lines of "Should the C standard start pinning things down more than it does?"

The answer is probably yes, but it's not straightforward to do. Look at just how much verbiage it took to create a decent memory model. And, still, using atomics in C (as opposed to C++) is brutal and the compiler can't help you very much.