C was created during a time where instructions were executed linearly with no vectorization, memory was a flat space with no CPU caches, and there wasn’t a branch predictor that may or may not execute the correct program branch in advance. The list goes on but the rest is beyond my scope.
C was designed for a now obsolete computer architecture model and over the years this old model has essentially become an abstraction that sits between C and the CPU. As such, C programmers aren’t really programming in their CPU’s domain anymore and C, by default, lacks the commands necessary to effectively utilize these new developments. It is left up to the compiler to translate C code from the old architecture abstraction into efficient machine code for our new machines.
For a more in depth look into this topic I recommend you check out (0).