because the parameter that determines size of black hole, per https://en.wikipedia.org/wiki/No-hair_theorem as in its Schwarzshild radius, is its mass. black holes that are not dense are supermassive black holes like those in center of galaxy, so it would just not fit. for example, black hole with density 0.64 g/ml has radius 3.3x greater than distance between earth and sun https://en.wikipedia.org/wiki/Schwarzschild_radius#Parameters

also it’s really bad for trout population