Because annoyance isn't fun, and games should be fun.
I'd actually be okay with this. These monsters are legitimate threats, so having them deny an area to you is less galling than having that area be inaccessible because it's filled with an infinite supply of one of the weakest monsters in the game. At least when hummerhorns are about, you know you need to tread carefully lest you be confused to death.
Alternatively the thing to do is get rid of lice, and make more things like Hummerhorns, Fireflies and Neekerbreekers. I rember something called a programming bug in ToME that might suit. Or Greater Wall monsters.
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