There's an old joke about that, the answer, because they can. But seriously folks, it may be due to some type of irritation. Perhaps a rash from being exposed to some irritant like harsh cleansers. May also be worms. Have the dog checked out to see if there is an underlying problem
There could be another reason. I just learned this the other day. Maybe it will help. Domesticated dogs have become over the years less able to release the build-up in their anal glands (sacs) voluntarily. They sometimes "scootch" their butts along the floor or carpet in an attempt to relieve the pressure. They will also lick themselves in the same attempt. A vet can take care of that and some groomers empty the sacs as part of their service.
It might itch or she might be cleaning it
Sounds like worms call your vet
Probably because it itches??