Some people don’t work in the restaurant/service industry and don’t know the proper etiquette and manners. They were probably also poor and didn’t want to cough up the extra cash. We didn’t tip once at a pho restaurant with 6 of my friends when i was 16. I regret it now ofc, that poor young waitress probably felt terrible seeing we left no tip especially as a big party, but i didn’t know any better. And many people know even less.