Why are people now worried about tiktok stealing their data, when Facebook, IG, etc. stole and sold their data for years?

It largely comes down to a confluence of a couple of things:

• The Chinese government has, on average, a much greater degree of involvement in and control over business operations of companies operating in China than is the normal case for most other countries in the world. I encourage anybody who doesn't believe this to go look up what the requirements are for an overseas technology company to legally operate within the Chinese market (it starts with needing to have an actual branch office in China, and gets more and more complicated and invasive from there).
• Throughout much of the West, there is a strong distrust of Chinese companies due to numerous events, ranging from poor/lazy QC leading to actively dangerous products, to blatant intellectual property infringement (or outright violation), to general business practices that would get them shut down in almost any other part of the world. IOW, we're conditioned to trust Chinese companies less than domestic ones. You can kind of see a similar effect in any country with trust of foreign companies, but a number of high-profile cases involving China have made it especially bad (though some of this is ironically a result of how closed off the Chinese market is, a number of cases of IP infringement are companies copying things that you just can't get in China).
• The Chinese government has been actively and openly hostile towards outsiders. They're behaving in an actively warlike manner without actually going to war (which is honestly much scarier than actual war for anybody who's thinking straight, because at least with a war you know what's going on), they take a hardline stance on censorship that means some of their citizens don't even know important parts of the history of their own country and means you can get arrested just for accidentally mentioning certain things, and they've regularly demonstrated that they don't care about the rest of the world unless it's beneficial to them to do so.

Taken together, these mean that most people who are not from China at best are wary of any Chinese company beyond the degree they probably would be for most other companies, and at worst are downright distrustful of them.

Its really a battle of the east vs the west. The west is considered the global market, heroes and so called righteous ... The east is considered as a secondary market, villains and inhumane. At the end of the day you can't trust any company with your data, encryption is key and especially, the 14 eyes. The 14 eyes are a group of 14 countries that are at the top of the surveillance capitalist disinformation marketing scheme. However, the bigger problem is that people are misinformed and don't take the time to learn about privacy. I wonder how many people took action regarding their online presence after the snowden revelations of 2013 or with the global quarantine of 2020. Thankfully, Privacy Tools is a great place to start.