Apple wants to earn more money by taking a cut from your transactions. Few people are ready to share their profits unless forced to do so. It's not monopolistic behavior, because Apple does not hold a monopoly in any market.
Both stores having the same exact revenue share may just be a consequence of there being a duopoly, or it may be more nefarious where active collusion has occurred between the two. Remember when Apple and Google got caught a few years back colluding against labor? https://www.cnet.com/news/apple-google-others-settle-anti-po...
Now, yes, Android does allow you to download apps outside the Play Store, but not by default. Microsoft was able to make a similar argument with Internet Explorer and Windows with it's monopoly cases. If you are one of a few large powerhouses like Fortnight or Tinder, you may be able take advantage of being able to bypass Play Store.
If I’m the only person that sells WalterGR’s apple tart appetizers, then I have a monopoly on that. But one could easily buy an them elsewhere...
Not trying to do reductio ad absurdum. One could think of an example between apple apps and Apple’s app store.