Because they're not community fish, they're fin nippers. They antagonise other fish to the point where they're attacking them as a play thing. Any fish big enough to eat them could die as they could expand in their mouths, and they do best alone, or with a couple of well selected tankmates, the only ones I can think of are Bumblebee gobies, armoured plecs and ottos.
They may seem fine in a community tank (or even with others of their own kind) when juvenile... as they get older, their aggressiveness will increase... odds are one day its tankmates 'will' suffer...