Instead, DoorDash would like you to think that you are paying the Dasher directly. DoorDash provided you a platform to find a Dasher and enter into a delivery agreement with them. And DoorDash is graciously "contributing" to the Dasher's income. This would make them no more an independent contractor than someone looking for work on Craigslist.
I don't think this holds up because DoorDash is custodial of the money before it reaches the Dasher's wallet, and because from the customer's perspective, they're ordering delivery from DoorDash, not a Dasher. But that's just my idle speculation.
I'm sure the service terms are different but in upwork's case, they're also acting as the escrow and taking a cut.