It’s basically an artifact of how pay is set. The USA has a system where pay for certain professions is adjusted only by a new law. Since in capitalism the capital class has power over policy and the working class does not, the tendency is to resist increasing salary.
Now for most workers this would simply be untenable, but for jobs that get part of their income through tips the workers can make up the difference by increasing the portion of their income they receive through tips.
So over time the tip rate has increased. It’s actually an interesting proxy for how fucked capitalism has become in the USA. The higher the percentage of cost that workers need to receive semi-formally through tipping, the more the imbalance between capital and labor.