I’d like to split this answer into parts.
First thing is that fast-food chains like McDonald’s tend to pay an inordinate time and effort at consistency. Within an area, they tend to be all either identical or very similar — you should have seen the uproar a few years ago when a particular McD’s restaurant failed to put ketchup (or put too much, I forget) in one kind of sandwich that people expected different.
So, if you *can* eat McDonald’s (or Burger King or Wendy’s or whatever), you can reasonably expect that going to one of them is going to be identical (or very close) to whichever you went to last time. Maybe that’s of no use to you, but to people who are sick and need to eat something safe that will not make them worse, that’s in fact priceless — I’ve heard from many people going through chemotherapy, for example, that at a certain point in the treatment they vomit nearly anything they eat, and the only thing they could stand (and ate for several days) was a McDonald’s Vanilla Milkshake and a banana.
Now, prices vary — there are countries (like in USA) where going to McD’s is relatively cheap and that’s why so many people who are on the low income side or just got someone to give them some money go there. Of course, there are countries (like Brazil) in which the reverse is true: McD’s is on the expensive side, and basically only rich entitled folks go there because, again, it’s clean and unlikely to make them sick.
Another reason to go to McD’s-like places is if you’re on a diet — I know people will laugh at me for this (particularly considering the documentary “Supersize Me!”), but if you are paying attention to the sizes you order, you can get a very balanced meal which has a good proportion of carbs, veggies and protein — I know a bunch of people with diabetes for example, who can go there and get a meal that will be nutritious and not break the rules; that is not true of everyone (people on insulin might differ), but it’s much better than a lot of serious restaurants that will serve you a meal with several times your daily allowance of salt, sugar and/or fat.
I’d like to touch on something that I don’t think other people mentioned in the other answers, either because they are way more polite than I am or because they may have not interpreted the question the same way I did.
I used to live in a household of half-a-dozen students or so, when we were of the age to go to University. One of my housemates was not necessarily rich in the way people tend to think of it, but on the upper-middle class, while the rest of us came from middle class or lower.
Well, wouldn’t you know it, at some point he started to rag on us about why we used to buy Stouffer’s Lasagna — according to him, “no Italian would ever eat such a rubbery foul-tasting thing!”… now, I took exception to that, and told him so: my entire family came from Italy at different times and from different places, in fact, some of my relatives were born in Rome, some in Bologna, some in Sicily, Milan, Florence, Turin, etc. And, while my own mother’s lasagna was the best of them (of course ;-) ), they were all edible, and at least a couple of my aunt’s lasagna were worse than Stouffer’s, so I figured Stouffer’s, just like any other industrialized food, were trying to appeal to the average consumer. I think it’s a bit too sweet, while someone in the Midwest might think it lacked a pinch of sugar… “Also,” I continued, “you might want to read the freaking directions on the package and try to follow them, because leaving the tray in the microwave on high for 10 minutes is sure to destroy nearly any dish, when we microwave the dish it’s quite edible!” — he never bothered us anymore.
Where am I going with all of this? I think you may not have eaten a lot of burgers if you are complaining that McDonald’s is not to your liking. I’ve eaten plenty of burgers that were much worse, particularly when people burn and/or overcook them on the charcoal grill and they are your boss so you need to eat it and like it. It’s fine, when you get older you’ll understand, no hurry.
And, no, not everyone needs to like everything, or the things you or I like. Some people like vanilla cake, some like chocolate cake, some like both, some like none. But you might want to work on the phrasing, because “why do people love cake, I hate cake!” or “why do people hate cake, I love cake!” tends to put you in the odd corner of the room. Not everything is about you and not everyone has to cater to your tastes only.
It’s very different if you are asking other people to take you into consideration (“I really don’t like McD’s, could we please pick another place?”, or “I don’t drink alcohol”, or “I have diabetes” or “don’t like cakes, can we add some other thing to the menu so people like me could have a choice, please?”) because people in fact do care about others in general and can help you.
For example, I just came home from a party where people who can not eat gluten (they have celiac disease), people with diabetes, people with allergies to certain foods, vegetarians, vegans, people who only eat meat (or it’s not a meal), people who drink and people who abstain all shared the same space and had a wonderful time because there was something for everyone and no one was making an effort to judge and/or put others down for the way they feed themselves.
Anyway, this has gotten already too long — have a wonderful life!