I've wondered about that myself at times. Some code that I've inherited was truly atrocious and much of that ended up rewritten, but other had been impenetrably dense. Those I had a mind to leave alone simply because I feared breaking the existing code. Usually we ended up "putting up dikes" around it and then clean it up piecemeal. Sometimes it is difficult to tell highly tuned and optimized code from a seething pile of cruft, but a quick run with a profiler against some performance benchmarks usually settles that question.
Oh and by the way, yes there is some code that I've written that I am somewhat proud of even if I recognize some of its shortcomings and ugly warts.