They don't really prevent new competition, rather than target markets that specifically have a larger barrier to entry such that new competition is rare to form.

i.e. these vet clinics - a PE firm can buy up 40 disparate vet clinics in a large city then raise fees and cut staff. You may be lucky if a few new clinics appear over the next couple years once customers are fed up with the increased price and reduced quality.

It's still a win for the PE firm and a loss for most of the consumers.