I think choosing one gig over another is a form of negotiation. The driver thinks about what will pay more: Delivering an Amazon Prime package, driving for Ube

I think choosing one gig over another is a form of negotiation. The driver thinks about what will pay more: Delivering an Amazon Prime package, driving for Uber, delivering a meal via Postmates, etc. So by choosing the one that pays best you're rejecting the others. So they have an incentive to pay more and get you to work their gig.