The main problem with SIMD instructions is that regular code doesn't use them. Almost always someone need to write SIMD code manually to achieve good performance, which is rarely done and if so, only in some tight loops and niche cases. Like cryptography-related code in a browser may be SIMD-based, but other code uses almost no SIMD.

Modern compilers are able sometimes to vectorize regular code, but this is done only occasionally, since compilers can't often prove that read/write operations will access valid memory regions. So one still needs to write his code in such a way that compiler can vectorize it, but such approach isn't reliable and it's better to use SIMD instruction directly to be sure.