Most multinational corporations have significant operations in China. Tech companies are essentially forced to unless they want to be locked out of the hardware side of the business.

Google famously left China, but still has offices there. It's impossible to ignore China if you want to develop the OS that runs on so many phones manufactured there (though I believe Google's own phones are built elsewhere). Apple obviously is completely reliant on China almost to a crazy degree. Microsoft caved to China on censorship to keep Bing from being blocked, they never left the way Google did. Facebook doesn't cooperate with China at all in their software products which are all banned, but they're increasingly getting into hardware and can't manufacture their hardware products without China.