Modifying the ABI of a function requires being able to track down all of the call-sites of the function, which is less trivial than you might assume. ABI concerns also tend to baked in relatively early in the optimization pipeline because you just simply can't get the ABI wrong, and I can think of several instances where the ABI decision causes missed optimizations.

There is also the other issue that a good algorithm for optimizing a problem like register allocation tends to be super-linear (e.g., quadratic), and if you shift the model from "allocate on a per-function basis" to "allocate all functions", the N in the O(N²) goes from "size of function" to "size of program," which is now suddenly a lot more compiler time spent for very modest gains. If register spilling across a function call is a noticeable component of runtime, then you're probably better off inlining that function in the first place!