Probably because
When you eat out, you normally don’t know what’s going into the food (unless you ask, but no one does that). So there for there could be things wrong with the food. For example: in most sushi restaurants instead of using tuna for sashimi they use a cheap fish called escolar which can cause different problems. But people don’t know that we are being severed this kind of thing.
Eating out isn’t necessarily bad
But you should do it all the time.