If somewhere charged a mandatory 20% “service fee” in lieu of tips, then why don’t they just up the prices 20%? I mean, sure the prices will go up - but not ho

If somewhere charged a mandatory 20% “service fee” in lieu of tips, then why don’t they just up the prices 20%? I mean, sure the prices will go up - but not how much the customer pays. 20% mandatory on $100 is $120. Increasing prices by 20% is still $120. What am I missing?