Because if every restaurant doesn't do it, the one that holds out from doing it and hides the extra cost behind a noticed-on-the-menu-later surcharge (or simply keeps asking for tips) will get more business because customers have trained ourselves to sort by lowest-price-first-then-choose-that-one.

> or remove the "restaurant wage" and force them to do minimum wage?

Several states and cities don't have the tip-credit restaurant wage and do require restaurants to follow minimum wage, regardless of tips. Tipping is still incredibly prevalent in those places because it's still a way for employees to make more money without the owner having to figure out how to pay for it through prices.