Because it tastes good. Fat, grease, sugar, and salt taste good to all people. We are biologically programmed this way.
The societal rejection of fast food has caused you conform to those standards, which causes you to feel disgust towards the food. In response to this internalization, you 'feel sick' after eating fast food. It's very similar to how everyone thinks that they got sick after a COVID shot, even though statistics say only like 4% of people feel sick after getting the shots. It's also the same reason why so many white women have can't have gluten and feel sick, despite not having celiac disease.