Because China doesn't ban US internet platforms, US internet platforms choose not to comply to PRC laws, which domestic PRC companies has to abide. Facebook/Twitter left because they couldn't/wouldn't censor calls of revenge killings during 2009 minority riots in PRC. It wasn't until NZ shooting and FB role in Rohigya genocide years later that political culture changed globally/domestically in US enough for FB to up the moderation game, around the time they wanted to re-enter PRC market. Except their employees protested and killed the initiative.

Flip side is Bytedance/TikTok bending backwards to follow US laws, because Douyin is used to dealing with PRC regulatory bullshit, meanwhile their employees just want to make money instead of undermine company expansion plans with geopolitical culture wars. Like it's not hard, follow the law in the country you operate in and be competitive. TBH that really leaves some Google services, a lot of western platforms simply can't hack it against PRC competitors for domestic PRC market.