Of all the services I've tried in this space, only Postmates lets you request items from businesses they aren't partnered with. This was a huge win once when I needed groceries delivered quickly.
The economics are even worse (because they don't get a cut from the restaurant), so these types of orders are either very high fee for the consumer, a loss leader, or both.
[1] https://gizmodo.com/doordash-pizza-arbitrage-shows-the-fubar...
[2] https://www.eater.com/2019/10/30/20940107/grubhub-to-add-res...
Also anecdotally, the probability of an order actually arriving and being correct seems to be higher on Postmates. Perhaps the drivers there are more motivated to do a good job.