In a SSE2 implementation, how should the compiler make sure that no memory accesses happen over the end of the string? It has to know the length first and this requires scanning the string for the terminating zero byte.
If you scan for the length of the string you have already accomplished most of the work of a strcmp function. Therefore there is no benefit to use SSE2.
However, Intel added instructions for string handling in the SSE4.2 instruction set. These handle the terminating zero byte problem. For a nice write-up on them read this blog-post: