Because the type of people who have no choice but to resort to payday lenders are the same type of people who need men with guns to visit their in their house at 2 am in order to pay back their debts.
Yes, they are common. Any industry that deals with primarily people who cannot afford to defend themselves has similar issues (e.g. slumlords). I have no doubt that they are profitable. My question was "why are they necessary to the business model".
Unless you assume "abusive collection practices" means threatening physical violence. Because I would assume it meant things such as chronic calling.