A lot of places' wait staff can make fairly decent money. They will usually make below minimum wage, but they often come out way ahead. So at some level, most businesses where you're expected to tip are using tips to subsidize wages. But there are some who take it to the next level, like IHOP or Instacart.
My mom was a waitress most of my life and made decent money, so I always tip 20-25% and a minimum of $5 unless I get extra shitty service. Depending on where you and your coworkers ate at, I would say they were just cheap asses.
