C compilers can and do generate SSE. Programs do rely on this code generation for performance. They fall back on assembly when codegen fails. That doesn't mean

C compilers can and do generate SSE. Programs do rely on this code generation for performance. They fall back on assembly when codegen fails. That doesn't mean that they aren't relying on the optimizations in other places. And a huge amount of graphics processing is always done on the CPU. Unless you're using Direct2D or something, rasterization is always done there (and even if you're using Direct2D, a lot of rasterization is still done there).