Tipping. I don't understand

What is an acceptable tip? Is 15-20% not enough? I see pissed off drivers when I leave a 20% tip for a 10 dollar order. That's 2 dollars + delivery fee ranging from 1.99-4.99. Is Uber not paying enough or are people just too entitled and want like 5-10 bucks for 10 dollar order that's 10-15 minutes away.

Edit. Thanks for everyone’s response and input!! I see both sides of the argument. I’ve always tipped based off percentage so if it’s a large order they may get like 10-30 bucks. But some times I just want a quick bite during a busy day and I order something that only gives a 2-3 dollar tip. Uber needs to pay more and charge us more instead of making it a tips as pay thing.