Many idle games are performing well in testing. However, once they are launched, the CPI increases very quickly and there is a drop in retention. Here is how it can be explained :
Idle gaming is not mass market : Idle games only interest a small part of hyper casual gamers. That's what we call a niche market. Once the few people interested in idles have installed it, the CPI starts increasingly quickly.
Idle games are not gameplay focused :The interest of an idle lies mainly in the metagame.
Not very feminine : You want to make games for everybody, not only for male players. Plus, advertisers spend more money on games that are appealing for everyone.