Yep, but if you look at the type of content that does well in each of them, it's not the same. Instagram Stories are known for have a "Behind the Scenes" mood while TikTok Stories are designed to reach a bigger audience. If you blend their content together, it will be a weird experience.
It's like blending a photos app with a meme app. Yes, they are all photo events, but each one has a time and mood during the day. If you blend them, they break each other up as a good UX.
