Use your common sense, anon. They take a base concepts and expand them. You have plumber guys, someone is in trouble in a building, fight creatures through various levels, use memorable tools that come back for each new game, introduce new creatures and characters each go, rescue who/whatever is in trouble, toss some shit humor in, boom. It's formulaic, and pretty much the exact same thing as you see with Marvel.
As to why it's FotM thing for art? No clue. Draw commissions of it to garner attention from your brain rotted customer base, and profit off it.