Two companies control the entirety of the mobile app distribution and mobile app payment markets.

If you want to enter nearly any mobile-related markets, you have no other choice but to have rent extracted from your business by either Apple or Google. Those two companies have stifled innovation in the mobile app market for over a decade now, and they expect you to pay tribute.

How the judge ruled that Apple is not a monopoly is beyond me.

Sure, but clearly other people are. As an outsider it's clear why people do, the risk drives away some of the competition (people like you), and I'll presume you are a thoughtful and skilled engineer. So here you have a situation where you have an increased risk of a rug-pull without recourse (maybe already a risk for you anyway), but plenty of money on the table, and good principled engineers like you choosing another route.

This was more about me seeing an unappealing risk profile than me being principled. The discussion about principles is a far more complicated one I think.

Fragile components make a bad developer.

Because it is a shit-ton of money. It is not like an app is going to be a multi generational sustaibale business. It will be on trend for a while, print money, and then be the next angrybirds or clubhouse (if you are lucky).

Chances are the users will have moved on before the users of the app have.

For some people it isn't their only revenue source and the adage "you don't leave money on the table" applies.