F-Droid gets what developers give them.
You can also get the Thank You App for all Simple Mobile Tools Apps for free there … https://f-droid.org/en/packages/com.simplemobiletools.thankyou/
OsmAnd is another example. OsmAnd+ costs money in the Play Store, OsmAnd~ (the same thing apart from the ~) is available for free in F-Droid.
If I got the FairEmail developer right recently, for all he sees the number of F-Droid users is currently negligible in comparison to Play Store users, so I guess there’s no big loss currently and no incentive for developers to think about this more seriously. And there might be idealists among them, too.