Because chain restaurants are generally kinda gross.
The typical American chain restaurant is going to serve you something that starts with low-quality ingredients, then gets a bunch of sugar, corn syrup, seed oil, artificial food coloring, emulsifiers etc added to it until it roughly approximates the flavor, texture and color of higher-quality ingredients (at the cost of being unhealthy). Then it has various chemicals added to it to act as preservatives or maintain texture or appearance during shipping and storage and re-heating. Then it's packaged into cheap single-use plastic bags or containers that slowly leach endocrine disrupting chemicals into the food, or rapidly leach them during reheating in a microwave.
The end result is that you get something that is edible, but probably kinda gross tasting unless your palette gets used to packaged foods and fast foods, and which is going to make you fat, depressed, distracted and tired.
There's a reason America has an obesity epidemic, diabetes epidemic, falling fertility rates and widespread prescriptions of drugs for depression, anxiety, adhd, etc, and it's the food the average American is eating.
The "horror" is less about going to the chain restaurant, and more the derision you'd face from your peers at suggesting it. The logic goes "If you like shitty chain food, it's because you don't have good points of reference to compare against, because you haven't experienced a wide variety of cuisine, because you're poor".
The “horror” is that you will only live to eat a fixed number of meals, so why on Earth would you waste one of your “meal slots” eating food at a chain that’s no better than reheated leftovers in your fridge?
Are you asking that question unironically? Why a lunch spent with your colleagues at CCF might be worth considering over the refrigerated leftovers waiting in the break room?
For me, it's like you suggest - more disinterest than horror. But it happens regularly with out-of-town coworkers. There are lots of above average ethnic places near my home and office (same 'hood). But, in a large enough group, there's usually one or two meat-n-potatoes person who doesn't want to try anything new.
