Why are most people are fat nowadays?

Bigger portion sizes being served, fast food chains everywhere, junk food being cheaper and easier than healthy food to get, junk food loaded with addictive chemicals that gives your brain a response similar to taking drugs, food loaded with hormones that makes you gain weight faster and lose it slower, etc.

Also as more medications are developed for things more folks are on medications in general, and a side effect of a number of medications happens to be weight gain.

There are many reasons. But generally, if you're an American, unless you like to cook and regularly make time for it, you are probably going to be overweight, unless you are very active, which most Americans are not.