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

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.

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.

Grease, sugar, and a lot of chemicals.

It's science, I don't know the specifics of craving but I know they spent a lot of money to figure out how give you that feeling.

"Because he puts an addictive chemical in his chicken that makes ya crave it fortnightly, smart arse!"

A very large culprit is your gut's microbiome (the bacteria that live in our gut). The more junk food you eat, the more you will grow your craving for it.

https://www.psychologytoday.com/us/articles/202303/where-cravings-are-bred

You haven't aged enough to where it feels like you have a rat trying to chew through your colon from the inside out when you eat it. That will cure you of the addiction right quick.

Want to experience the ultimate in shitty comfort food?

Come to the UK, go into town, have a few beers, get drunk and then stop for a massive dirty kebab on the way home.

Fucking bliss

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

https://en.wikipedia.org/wiki/Bliss_point_(food)

fast food isn't bad for you, unless you eat only fast food for years. if you wanna recreate the MCD taste at home, try sprinkling MSG on your food.

Crack tastes like shit too, and it's maybe even worse than McDonald's

They has test kitchens and spend a ton of money on R&D to create that craving plus all the cartoon characters and kids toy’s and sponsoring school events gets kids hooked young.

Because we didn't evolve to live in lands of abundance. We evolved in scarce conditions and are attuned to that.

Food that delicious and high calorie would not be constantly available in the wild, so if we ever found anything close, it would be beneficial to eat as much as possible to store up calories and survive during periods of scarcity.