One word: variety.
My local sushi place isn't going to have enough delivery orders to justify having delivery drivers on staff.
Heck, I remember years ago (Was it the late 90s?) when McDonald's tried to do delivery.
But yes, their business practices suck, and that's our fault as a society for yet again allowing capitalism to run amok.
The problem of course is that a restaurant that refuses to work with deliveries then ends up getting significantly fewer orders than ones that do. It creates a bit of a racket. Restaurants feel forced to cooperate, or lose significant business.
As a customer, I get around a lot of the extra costs because my credit card gives me the Dash Pass for free as a little bonus, and Costco sells $100 DD gift cards for $80, so I'm effectively getting every deliver at 20% off.