We got my son a Mac Mini when he was 6. I was surprised at how many kid games just didn't work with the Mac, or how many did work but didn't support an external microphone and camera. I guess since most young kids have iPads or Chromebooks there's no market.
It makes it easier to pirate your app if you enable that checkbox. macOS attempts to disable iOS apps when SIP is disabled to prevent this but it's not difficult to bypass [1]. I don't necessarily agree with it but this probably does factor into their decision process.
That couldn’t be the reason. 90% of App Store revenue comes from in app purchases of consumables from games. This came out in the Epic trial.
The rest of the most use apps are front end for services where the app is free. There are very very few one time app purchases on iOS where pirating would make sense
They're encrypted using FairPlay, so you need either a jailbroken iPhone or a "jailbroken" (SIP bypass while SIP is enabled) Mac to decrypt them. The former will stop being possible soon enough, the latter will likely remain possible for quite some time.