One possible reason is they need a carrot to get people to switch to their engine on other things, the carrot is they're offering rebates if you switch to their stuff in some areas (and stop using competitors).
Of course, they'd probably end those rebates as soon as AppLoving or the other things are no longer a major threat.
Unity is offering a Runtime Fee waiver if you switch to LevelPlay as it tries to "kill AppLovin" - Mobilegamer.biz