Because the tradition started by generous people who would be like "Oh hey, thanks for serving us kindly. Here's some extra dough for you!" The employers had nothing to do with it initially.
people started tipping one profession in particular because they felt like doing so, capitalism just adjusts for that by paying its workers less so the firm can stay competitive with other firms in the industry
I'm guess it started in the first place because waiting tables is more of a personal service than like changing a tire. The waiter takes your order brings your food and makes sure you're properly taken care of. You're interacting with them a lot, so some people might be more inclined to give a little extra to someone that does a good job and was generally pleasant. So they kick in a couple extra bucks. Then some restaurant owners went "Hey wait a minute!" And cut pay because fuck it it's almost like you're getting a salary anyway! And it became in institution.