My understanding is that a large number of Unity installs are mobile games, and a lot of those are free to play with microtransactions. So we're not talking $30 earned per user. We're not even talking $1 earned per user. We're talking, in some cases, less than $0.01 earned per user (on average), since some users have paid nothing to play the game.

There's also the fact that Unity charging "per install" also includes things like game demos (which are free), or games given to subscribers of things like PlayStation Plus, where the creator is paid a fixed amount upfront based on an estimate of how many people will download the game--but, if more people download the game than originally estimated, then you could end up losing money thanks to Unity.