Because sugar, fat and salt in those quantities are literally addictive. Our human/monkey brains were wired for survival, not overall health. Eating a large source of those three would’ve been a massive help in surviving winter when our next meal wouldn’t be guaranteed.

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.

Grease, sugar, and a lot of chemicals.

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

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.

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

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.

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?