They're trash because they're free.
Traditional games need to be good, so people buy them.
You don't need to buy mobile games. But developers need to eat. So the money needs to be extracted from the people while they play.
So you need to implement microtransactions and design the entire game around making them necessary for success.
But most people stop playing a game at some point when they've beaten it, or are getting bored ot it.
So you need to make your game addictive instead.

The same principle applies to so many things (for example news).
If you don't pay for it up front, the entire thing will be designed around extracting money from you during use.
Which means it needs to be designed to draw you in and keep you addicted. Delivering quality content is literally worthless.