I think it has less to do with any culture, and just how the pay structure works for all the different things you mentioned. USPS/UPS/Fedex/bus/metro/train drivers all make livable wages and have benefits.
I'm not defending the tipping pay structure, but it's certainly not some confusing mystery.
I prefer to order food from the restaurant directly and pick it up, so that they get the maximum $. Door dash is convenient to see what's out there, but not very good to the actual business of the restaurant. Plus you are paying for that convenience. I also don't see how DD will ever be cash positive as the fees would have to go up, which seems bad business.
In many cases there's a markup on the food over the restaurant's prices. There's delivery fee that's generally $2-$3, a service fee that's hidden in the "taxes and fees line item", not to mention a small order fee etc on some orders. And, of course the tip. This I do not understand. It went from tipping the waiter a bit more because you were well taken care of at the restaurant dining experience, to pre-paying a tip to the driver before knowing if the food's good or if the delivery is late etc. The restaurant doesn't get any of the tip. Anyway, a subsided food delivery that you got is not sustainable.
To be honest I do not understand the tipping culture at all. Its expected to tip a restaurant staff if you eat at the restaurant, but not if you get the food delivered. But, expected to pre-pay a tip the driver who delivered the food (irrespective of quality of food or delivery) but not the USPS/UPS/fedex driver who delivered anything else even if he/she carries a heavy package over a flight of stairs. Its expected to tip a uber/lyft driver even on a shared ride with other folks but not a bus/metro/train driver. None of it makes sense to me :)