DoorDash has a model for what it considers "fair compensation" for an order. Since the company can see exactly what the customers tip in advance, they'll just subtract the tip amount from their previously calculated "fair compensation" and show that base pay / tip breakdown to the Dashers. DoorDash isn't going to suddenly increase its Dashers' pay by 50%.
If you want to actually compensate the Dashers, tip $0 in the app (raising base pay) and hand them cash at the door. That seems to be the only way to get around this policy.