> I wonder why they didn't buy TikTok when it was starting to become popular like they did with Instagram or Whatsapp.
ByteDance was a large company before TikTok and had other successful products. TikTok actually developed out of a billion dollar acquisition Bytedance made[1]. It wasn't for sale. Sort of like how Tinder wasn't for sale because it was owned by IAC (Match, OkCupid).
Also regulatory environment in the US and in China would make it unfeasible.