The same reason people will logically understand that crack is bad for them but crave it like crack

You crave salt and fat because your body needs a little bit of these things to survive, but finding salt and fat out in nature is really really hard, so those cavemen that liked the taste of salty or fatty foods enough to make the extra effort to find those foods were more likely to survive to be your ancestors and you inherited that behaviour. That’s why you like McDonald’s, it’s full of the salt and fat that is hard to obtain if your diet consists of mostly roots and mushrooms and leaves.

McDonalds is bad for you because it’s unnaturally full of salt and fat. Far, far more than your body needs and far more than your cavemen ancestors would have eaten naturally. Especially if you eat McDonald’s often. Too much of anything turns that thing into a poison.

McDonald’s has only been around a generation or two. That’s not enough time for the people who crave McDonalds and eat too much of it to die off, leaving mostly people who don’t crave McDonalds to remain.

Because your diet sucks. A few months of eating only unprocessed food and you’d probably eat McDonald’s exactly one more time in your life

And I do specifically mean McDonald’s… It’s uniquely disgusting, even among fast food. It tastes like it’s reconstituted food scraps doped with sugar

I don’t get it.

I’ve tried McDonalds food.

It’s more expensive than much better alternatives, it doesn’t look good, and it tastes worse, like the cheapest ultraprocessed crap you can find in the kind of budget supermarket that only carries foreign brands you’ve never heard of.

I can find better and cheaper food in seedy bars I’d never willingly go to, or by buying the cheapest brands (even the good brands or fast food joints, or buying natural ingredients, wouldn’t be significantly more expensive) and cooking at home.

It really boggles my mind. Is it like smoking? Do people start eating it due to peer pressure and never stop because they get addicted?

When I was a kid, I heard two unsubstantiated rumors about McDonald’s food:

1. It used kangaroo meat (not sure why this was a claim)
2. It mixed in nicotine (to make the food more addictive)

I doubt that either are true, but if the latter were maybe it would answer your question.

There is a chemical in your brain called dopamine which is an important part of how we feel pleasure. Use of recreational drugs and alcohol causes a rush of this chemical and that is part of the pleasure we feel from using them. The problem is that regular use of such chemicals causes us to have lower levels of dopamine when we are not using them. We end up feeling a desire for the drug or booze to get our dopamine levels back up.

Diets high in sugar, salt, and carbs also causes a dopamine rush. When you eat that food regularly, it lowers your normal dopamine levels, just like drugs and alcohol do, if not to the same level. That is why you feel that craving. Eating such food occasionally is fine, but if you do it to often, you can literally get an addiction to it.

Edit to add -

You mentioned that fast foods are low quality and bad for you. That is true, but but only in high quantities. Fats and sugars were high value foods to our ancient ancestors. Fats are very high source of energy. You get more than twice the energy from a gram of fat than you get from a gram of carbohydrates or protein. Sugars are easily digested making them a source of quick energy compared to other carbs, fats, or proteins. Eating these kinds of foods gave us a survival advantage over those who didn’t, at least until we learned the agricultural skills to make them easy to acquire. Now, many or most people can get such foods any time they want and though they no longer give us an advantage, and eating a lot of them is actually harmful in the long run, those ancient taste preferences still remain in our evolved programing.

Because fat and sugar are drugs. People don’t usually think of them like drugs, because of the widely accepted, but very wrong, attitude that only illegal substances can be drugs. Sugar, fat, caffeine, alcohol or tobacco are all drugs. They all trigger a desirable chemical reaction in your brain, and all have addictive potential. At least 2 or 3 of them are also significantly more harmful to you than several types of actually illegal drugs.

And in general, though perhaps most strongly for drugs, many people suffer from the cognitive bias of “illegal=bad & legal=good. Automatically and by default”