I don't know for sure, and I certainly don't think anyone who wants to keep their job at PayPal (or at eBay, for that matter) is inclined to give up any secrets. So my best educated guess is that they collect interest on the money that sits in their coffers. In a way, we would expect that because we live in a predominantly capitalistic society. It makes sense, therefore, that the service PayPal provides (or believes it provides) us has a justifiable revenue system built in for PayPal.
They already make a ton of money off of commercial account fees. All power to them for making money in other ways.
I suppose the logical thing to do is have PayPal sweep money into your checking or savings account on a daily basis.
Good luck to you, and continued success on HubPages!