There are already libraries that make it easier to do vectorization, which are equivalent to compilers that take generic instructions and convert them into SIMD equivalents (though it's somewhat more nuanced than this). The issue is not so much the compilers, but rather having a standard language for describing how to do the maths on vectors.

It's a lot like the difference between writing C or assembly. Even today, sometimes you need asm to make the code go real quick because what the compiler spits out isn't always optimal (but for the general case, it's quite good).