I can't speak for AA (or UA or DL), but there is zero reason for it, it's a hassle for the customer and I suspect a hassle for the carrier in those instances where people prepay, then cancel and whine about refunds and the like.

There is some argument that prepaying might knock 30 seconds of the interaction time with an agent / kiosk, but then you get people who paid for 1 bag and now have 2, who paid for 1 and now it's overweight and so on.

Me? Unless there was a huge discount for prepay, I can't conceive of doing it.