As others have said, it's for more or less the reason you'd expect: many are scammers, perverts, or rude.
So long as you report a scammer quick, they'll have their account deleted within a few hours. Perverts and rude people will have their accounts deleted pretty quick as well so long as enough people report them.
People deleting their accounts can be another factor. I think at one point there was a requirement that an account be at least a week old before it could be deleted, but I don't know if that's still the case. Usually it's just because they were obviously catfish or scammers, or they were sending an onslaught of rude or perverted messages to everyone they could.
Some people will think this means the mods are ban happy, but this is actually a good thing. You don't want to be on the site if it's filled with scammers and people with little-to-no decorum.