It is more that the cost of operating a dining area isn’t worth it for a lot of fast food restaurants.

You have to dedicate staff time to maintaining the dining area. You also have have to maintain a public restroom, even if it is only for customers. At best, you only get the random biohazard of someone shitting themselves. It then slides to being the bathroom and living room to an unhoused population or a place where junkies go to do heroin.