There is a psychological term for it but essentially we as humans are fucking dumb when it comes to money and even though we tip, a higher price on the menu hits us differently than a lower price with a 20% tip later.

Iirc the experiment was done under different types of service as well and consistently across the board people preferred to see a lower price even though the overall cost was the same.

If you already tip 20% and don't complain about it you're probably not one of the same people who don't tip or tip minimally and complain about the price. I've gotten 50 cent tips on 30 minute cab rides while being paid half of minimum wage by people who complain about prices constantly. There's a pretty wide range of attitudes and behaviors around tipping and treatment workers.