An STL implementation has a few things going in its favor:
STL implementations use best-of-breed algorithms.
The designers of STL implementations are, more than likely, domain experts.
Those domain experts were entirely focused on the mission of providing a flexible, powerful, and efficient library. This was their primary task.
But then again. (only) You know your own usage scenario, data types and platform. That knowledge can probably offer some profound short cuts and optimizations.
Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (7) Well, it's an excellent idea, but it would make the compilers too hard to write.