Tipping isn’t mandatory. But the issue is if a lot of people stop tipping all at once, servers will quit those locations. Then those locations will almost certainly go out of business because they can’t afford to pay a living wage because the US’s commercial real estate is insanely expensive. Current restaurant models essentially are built on this dynamic and to change it would require a lot of moving pieces to change. But for those pieces to change, a LOT of businesses will need to go out of business all at once to tank the real estate market.
And you may think: if they can’t afford to pay a living wage, they should go out of business. That’s a reasonable stance but it ignores the result: megacorps will buy up real estate and only huge chain restaurants will likely survive these kinds of busts. All your local favorite places will go under and be replaced by Fridays or Applebees because their thousands of locations can close 25% to focus on profitability.