Yes, there is a reason. Microsoft and Apple want to make money in CJK countries. And they have architects that make system-wide decisions.
That is not how Linux works. Companies contribute where their business is. That is server or embedded. After Canonical closed bug number 1 https://bugs.launchpad.net/ubuntu/+bug/1 no big player is interested on Linux on the desktop anymore. Individual contribute what they are interested in and what they know best. I fear most Westerners don't understand the challenges of CJK and other more "complicated" scripts. I myself "blame" Americans if they do it wrong and something accepts only 7 bit ASCII. I can fully understand if CJK or right to left people blame us "8 bit people" (character set, not coding) for doing it wrong. We just don't get it, that's a fact. But I don't think studying Korean etc. is a realistic solution. The only way to change it is to have more people and companies contribute that a) need it and b) really understand the user needs.
Yeah, but every app would 'just work' if we have a level of indirection with an IME by default, even for languages with Latin characters.
I mean, there is a reason why Windows & macOS all selects a similar architecture on text inputting.