Well, flies and all sort of flying bugs are very good at getting into homes and very bad at finding a way out. They stick on a closed window and can't find the open one next to it.
There's no genetic advantage to "finding a way out"! The home barrier way in is a genetic hurdle - flies that cross it are free to reproduce in an abundant environment. This calls for a "quieter" fly (a stealth fly?) who annoys the local beasts minimally - yet another genetic hurdle.