As someone who grew up in the US, from my perspective it’s less a question of “how good of a job did they do” and more a question of “did they do so poorly that I’m okay with them not making enough money to pay their bills or buy food this week.” Not that my single tip is going to make that difference, of course, but at least in my circles the thought is that delivery drivers and waitstaff are paid poorly enough that tips are needed even for average service. It’s not a great system and I’m all for changing it to making tipping truly optional, but in the meantime I’d rather tip even subpar service than contribute toward someone’s financial worries.