there is also something very simple:people do not like feeling like a cow that people milk for money.
Some people already feel (wrongfully so) that since they pay their internet connection they should have everything on it for free, others of course do not feel that way and know that paying for a product helps the ones that did it, but when they see ads on top of the game they paid for, it is just "We know you paid for it, but we might as well make more money so sorry but enjoy the ads", not sure about other countries but pretty sure that this kind of app would be considered either as a lack of respect/big middlefinger, or as unprofessionnal ("did they just forgot to remove ads before making it paid? are they incompetent or just stupid?", sure, it is rude to think like that but thta would be my first reaction).
it is regarded as a bad practice (ads themselves on a free app are often regarded as such too), so be sure to understand that first.
Game design is all about decomposing the core of your game so it becomes simple instructions.