One prominent example: the use of intermediate representations based on basic blocks introduces redundancies that increase the complexity of the compiler, requiring attendant redundancies in order to optimise the same. You can see the redundancy manifest here https://godbolt.org/z/8o3oe39hh as different code generation from f and g. (They may change the result of this particular test in the future, but it seems unlikely that the disease—rather than the symptom—can be treated without a complete rearchitecture.)

E-graphs ameliorate phase ordering issues and allow for exploring the space of non-monotonic rewrites; recent research makes them computationally viable.

Put simply: it's legacy. Gcc and llvm are millions of lines of code, and they assume a particular architecture. Changing that is not easy.

Another issue, which I did not mention (but which is pertinent) is that c is a poor language for compilation. (Fran allen famously said 'c has destroyed our ability to advance the state of the art'.) In some respects, the optimisations performed automatically by modern high-performance cpus are more sophisticated than those done by c compilers, howbeit with less reach; the only reason they are able to do this is that they have direct control of the execution and hence have a greater ability to abstract over the side effects which are rampant in most c code.