American cheese is more properly called "reconstituted cheese." It starts out as cheddar that gets reheated under pressure with preservative added. Velveeta is effectively the refried beans of cheese.

Now that irradiation is an everyday workaday part of dairy processing it's hella easier to make any cheese shelf-stable long enough to sell at the grocery store. The reason we all ate Fucking Velveeta is that it was the only goddamn cheese that lasted longer than a week. Now? Now I can buy like fifteen different types of sliced sargento, not to mention that whole frickin' island of cheese from like Greece'n'shit.

This has exactly fuckall to do with millennials. nobody will eat Fucking Velveeta if there are other choices. 'cuz you know what? Laughing Cow is doing just fuckin' fine, thanks.