They involve too many highly questionable paradoxes for one thing. Otherwise to put my understanding of physics simply, if ordinary matter cannot undergo a transition to the speed of light, it seems illogical to propose that any combination of ordinary matter, with finite density and finite gravitational pull, could ever make the transition into an object with infinite density and infinite gravitational pull. The layout of a time-space field would endlessly warp before ever allowing any sort of "event horizon" to exist that is capable of infinitely transforming a photon's wavelength within that same field. In my opinion, black holes are mostly a kind of sensationalist idea. If any similar kind of object exists in our universe (which is still a question for me as well) it would probably have finite mass that is contained within an area of greater-than-zero size.