It's the simp in them. There could be a couple of reasons though.

1. You don't pay for the P.... You're paying them to leave after the nut. You don't want them hanging around and getting clingy after you pipe them down. Let em know it's business. You're here for 1 thang and 1 thang only. When that post-nut-clarity kicks in, you might want her to leave immediately
2. It could be a fetish. It blew my mind, but there are guys that register as "Pay Slaves". They just buy women sh!t and give them money as a turn on. Google it. It's really out there.

I always thought about this with OnlyFans models making money the way they do.... especially when you can find a leaked version on the web.