I think people don’t understand your question, or think it might be sarcastic. So to answer your question: server storage and computation power costs money. Depending on how your app’s backend works, this can be cheap or very, very expensive, paid monthly or yearly. It also needs to scale with the number of your clients actively using the backend. Some of us just sit on the costs to give its users a free and ad-less experience with more functions without taking any money (by the thought “I pay for this server anyways, so why not share it”). But it costs me more, if I have more active users and I have to actively compensate this.

But there are also some greedy bastards, taking much more thinking to get rich with a single app (actually met one of this devs)

The average user doesn’t work like that, also an average user does not always think he is average. There are many people thinking they are advanced, because they know where settings in Windows or Android are located. You will probably get bad reviews then emails, because quote “your app doesn’t work”. This comment is based on real experiences with Google Play Store and its users, thinking they know what they do.