They spend years preparing to squeeze out all comers in particular categories, except one.
Nobody wants to go there.
Until humankind began flitting around in space, the only black hole we had heard about was the historic, murky dungeon in Calcutta, India. But, now that we are out there probing around, all sorts of phenomena have popped up. Some of them we know something about. Most we don't. We do know a collapsed star is what causes a black hole, but no one can explain what is inside the hole.