I cannot wrap my head around tipping culture

Employers don’t pay servers enough, so there’s this social expectation that the customer is going to make it up. But instead of adding an appropriate tip to the bill already, they frame it as optional? Why? And you’re rude if you don’t do it, but you’re still rude if you don’t give enough. Again, why?

This is all so stressful. Why am I the one who determines all of this?