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.
Anecdotally, it has made things worse. The shopping bags were already being reused as garbage bags, now I have to buy rolls of single use plastic bags instead. Worse, those tote bags are everywhere now, and so much less ecological.