This kind of crap is why companies just aren’t trustworthy without reasonable regulation. They default to “trick the customer” every chance they get.
Even before apps, e.g. back in the day when ATM fees kept increasing, I kept doubling the amount of cash I took out so that the fee change didn’t matter.
In general I think businesses count on most people not noticing or finding it too inconvenient to change behavior. Probably the same principle behind forcing people to spend hours on hold to make companies do things they don’t like, such as cancelling things.
They probably don't. They probably don't realize that the food is marked up in addition to the fees.
I would imagine, most users look at the $5 fee, decide that's worth delivery, and assume the prices of the food are the same they'd be paying to the restaurant itself.
If you're making minimum wage, your time is worth a few cents a minute, driving may be a good deal! If you're more highly paid and making ~$2/min post-tax 15 or more minutes may be "overpaying" in time, so paying for delivery makes perfect sense.