Why are Apple users so quick to recommend me Android when I point out this particular flaw with iOS? And yes, I do consider distrusting the owner's software installation preferences to be a flaw, especially when Apple is certainly capable of maintaining a robust and safe third-party distribution ecosystem on macOS. The Mac isn't demonstrably less secure because you have a sanctioned way to install Steam onto it. Likewise, Apple doesn't lose the ability to maintain the quality of the Mac App Store because people might get their apps on Steam.

"Safer for kids" isn't a factor here; Apple already provides tools for parents to lock down their children's devices should that be necessary. This would also presumably have an option (or default) to disable third-party apps, in the same way parents can already disable specific App Store and built-in apps.