Because making amazing looking games is, believe it or not, easy.
You add a lot of shit on top of other shit and WHOA bells and whistles and dragons and flying tigers!
Then you notice the box that is running the games can't calculate and render everything fast enough. Turns out making your game look good enough and run well enough is the hard part.
But more than incompetence (which I don't rule out), it's marketing: people will just look at the screen and think "that looks cool!". They won't be able to understand what the framerate is without a direct comparison.