Reason I don't use these services is the tip is expected in advance. Bring me my food hot and in a timely fashion and I'll give you a good tip. I'm not paying you ahead of time to take too long bringing me cold food. I'd rather pick it up myself.
So little of that is actually under the control of the courier. I did that for a little bit, and generally the drive from the restaurant to the customer was the shortest part. You get an order for a restaurant that hopefully you're close to, but maybe you aren't. You get there and maybe the food is ready, maybe it isn't. Maybe it's been sitting on a shelf for half an hour. Maybe traffic is heavy, maybe the GPS gives you shitty fucking directions. Maybe you ordered from the wrong restaurant and instead of being 5 minutes it's 20. Sure, there are shitty couriers, but 9 times out of 10 your food is cold or late because of circumstances outside of their control.