You're guess is wrong. :P And anyways, I didn't say all games using an easy to use game engine are shit.

If you use an easy game engine (idk if unreal would even fit this, btw), it is easier to produce something usable at all. Meanwhile, the effort needed to make the game good (i.e. game design) stays the same. The result is that games reach a state of being publishable with a lower amount of effort spent in development.