Your basic theory of "app developer wants me to like the app" is flawed. They don't want you to like the app, just to watch as much ads as possible. And since they get more money if you click the ad, the X has to be extra small. But they cannot remove the X since guidelines say they have to provide a way to close the ad without interacting with it.

Why don't you stop playing f2p games? There are proper games from reputable developers.

I would prefer to believe it's the latter, but it may also be a design choice that merely reflects a general disdain for the user, rather than being motivated by actual greed or cynicism

They are using it to lie about the clickthrough rates

This is why I have like 4 apps specifically designed to strip out the built in ads of other apps.

Oh, they know you hate it, and they're currently thinking of new techniques that will piss you off even more.

They still get paid.

Good way for me not to care even a pixel of these games.