To be honest though, I’m less concerned about app portability as I am about retaining ownership of apps after their or the app stores support has ended. While this doesn’t really matter for mobile apps nor music streaming services, it’s something that really concern me with regards to collecting retro games. There is a lot to be said for owning physical media. But I’ve digressed.
For example I use Gimp and Inkscape, irregardless of the OS. The experience is a little shitty on MacOS, but they get the job done.
It's also the reason for why Microsoft was able to provide the "Subsystem for Linux" on Windows 10, because the Linux kernel is much easier to mock than Windows is, being a much smaller effort than what the Wine project has to handle. Speaking of Wine, it has been doing a decent job of running Windows apps on Linux, many Windows-only games are playable on Linux due to it. And Steam has been using Wine to make games available on Linux.
iOS on the other hand is another matter entirely. First of all you don't have direct access to the binaries, without hacking the OS and Apple is doing everything they can to make that OS unhackable.
No, we are not talking of technical limitations, as those aren't insurmountable.
You do realise that there are technical reasons for that.
You can't just lift an iOS, Linux, Windows, Android, PS4, Xbox etc app and just run it on a different platform. Especially if your target platform is a resource constrained one.