Earlier quoted context omitted.
Even if they did, creating a Chinese IME is a really difficult problem space. Since typing is generally done phonetically in mainland/HK/taiwan and the real words being typed are ideographic, there's a lot of inference to translate in between. Considering how quickly the language moves to keep up with internet culture and new newsworthy names, new parlances, new memes, an IME has to do the equivalent of staying up to…
The side effects of chinese not adopting a phonographic writing system when it could, or having both like japanese does so you don’t need to use such an informal ambiguous layer when actually writing.
Imagine typing English by speaking. You can say "flower", but that might get written out as "flour". Some intelligence has to be implemented that picks the right one based on context, or give you the ability to correct it. That is where the complexity in east asian input methods come from.
(Yes, as English-speaking computer users we are very lucky. The exact sort of symbols that readers of English expect map 1:1 to our keyboards. Still kind of a pain on a phone, though!)