It would be no problem whatsoever if playstore/appstore were just two of many market places. As it is however, they both completely dominate their respective markets. On top of that, it is costly, both in terms of money and in terms of friction, for a customer to change between the two, so they really are quite separate markets. As a customer, you pretty much choose one of the two, then stick with it (or pay a hefty price to switch). As a developer, you either also choose one of the two, or both. Choosing neither really isn't an option.
In this situation of non-competition, some very crucial assumptions about the efficiency of free markets break down entirely. I think hardly anyone is debating that apple/google make some money for the infrastructure they provide. But the amount they take would be significantly lower if there was real competition. Which there really isn't. That's what's called "rent extraction" and basically just means you're extracting more money for something than would be needed to make you do it in the first place. Would google/apple still be motivated to provide the same service if their cut was halved? Probably. It would still make a killing. But their dominant position allows them to take more, so they do.
TL;DR:
At the base, your question (for me) really boils down to "why should there be any rules for corporation at all?"
To which I would answer: because if left completely to their own devices, that would be a bad system for nearly everyone, except the very few at the very top.
In this situation of non-competition, some very crucial assumptions about the efficiency of free markets break down entirely. I think hardly anyone is debating that apple/google make some money for the infrastructure they provide. But the amount they take would be significantly lower if there was real competition. Which there really isn't. That's what's called "rent extraction" and basically just means you're extracting more money for something than would be needed to make you do it in the first place. Would google/apple still be motivated to provide the same service if their cut was halved? Probably. It would still make a killing. But their dominant position allows them to take more, so they do.
TL;DR: At the base, your question (for me) really boils down to "why should there be any rules for corporation at all?" To which I would answer: because if left completely to their own devices, that would be a bad system for nearly everyone, except the very few at the very top.