As a European, I never understood why you'd tip automatically. I get that waiters are allowed to be paid less, but I don't see why that would be the customer's problem.

Because it's part of our culture and it's an easy way to show appreciation. You don't have to do it anywhere. Waiters also explicitly aren't allowed to be paid less. They must make at LEAST minimum wage. If they don't make minimum after tips, their hourly rate is raised each paycheck to equal minimum wage. It's literally not the customer's problem. You should probably learn how things work here if you're that curious!

I also don’t get the logic of tipping a percentage of the cost. Asking for several cheap beers is more work for the waiter than a single bottle of expensive wine, yet the latter earns them more?