Linux apps are easily ported to other platforms and this is because the Linux kernel has an easy to port interface and all the GUI toolkits, along with the X Window subsystem are open source and have been ported already.
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.