I think a simple love of complexity is also part of it. It's fun to do things that make you feel clever.
One experience I've had more often than I'd care to admit is diving into a multithreaded module with the intent of fixing a race condition bug, and finding that I could simultaneously remove the bug and realize a healthy performance improvement by making it single-threaded.
Which, for that matter, is another reason to be wary of mutexes and shared mutable data: Memory barriers do really impolite things to pipelines and caches in a modern CPU.
One experience I've had more often than I'd care to admit is diving into a multithreaded module with the intent of fixing a race condition bug, and finding that I could simultaneously remove the bug and realize a healthy performance improvement by making it single-threaded.
Which, for that matter, is another reason to be wary of mutexes and shared mutable data: Memory barriers do really impolite things to pipelines and caches in a modern CPU.