There is a very clear different from the consumer's point of view between paying a 20% delivery fee with no tip (and realistically having most drivers still expect an in-person tip) and a 5% delivery fee with a tip amount of your choosing (and maybe only 1/1000 drivers expecting/requesting yet more).

I have always been a generous tipper and I try to always put myself in that person's shoes when deciding how much to tip but even I notice the psychological difference. If you don't ever think about it, have never had a job waiting tables or dealing with the public, it'd almost be a nonstarter to have a higher delivery fee regardless of tip expectation.