While you are right, we are where we are and there is tremendous leverage in making an extremely common (even if undesirable) thing more efficient. It kind of doesn't matter why millions of lines of code perform memcpys - they do, and we're unlikely to change majority of them anytime soon.

P.S. I work on the type of systems where we obsess over unneeded memory accesses and copies. Minimizing those is not as trivial as it may seem, especially if you care about memory safety.