Also, the thing they're spending money on is the hard work of other companies, not Google's. The profit margins are not tied directly to a Google product; the Play Store generates very little of its revenue in-and-of-itself (Play Pass is the only thing I can think of).

The Play Store certainly isn't cheap to maintain but it pales in comparison to the amount developers collectively have to spend to create/maintain their apps. The Play Store's profit margin is obviously going to be high, because Google doesn't have to spend much to get a cut of the revenue from other companies that have spent quite a lot.

And that's sort of to be expected with any digital store front that manages in app payments. The question is how much of that profit goes back into the Play Store, or Android development, or into other Google products, and how much does Google eat into the profit margins of those other companies while preventing them from managing payments themselves.