ByteDance is valued at over $80 billion, if the Chinese government would even allow the purchase. They've also invested a helluva lot of machine learning resources into building a platform that's hard to copy.
Hard to copy??? It's 30 second videos. They don't have any amount of machine learning or tech that FB doesn't, and in this case we're just talking about user recommendations, which FB is the absolute master of (perhaps on the level of Google, but no one else is close to those two).
If it was that easy, Tencent would already have copied them. Google's video recommendations (at least on YouTube) are absolutely horrible, and Facebook has almost zero experience recommending videos. Effective recommendations really make a difference; that's how ByteDance's first product, Toutiao, a news aggregator, became so popular.
People use youtube for more than 40 minutes at a time on average, FB; 20 minutes. These are because their recommendation systems are, counter to your opinion, really good. Facebook's whole business depends on presenting posts based on how likely you are to engage with them. Other companies might not want to copy TikTok right now because it brings in a whopping $3 million per month, so probably not worth it. There are more reasons behind not copying a business than the ease of doing so.
