I mean, a video player is actually quite stateful. Buffering video data and then decoding delta-changes to get the current image, while adapting to varying connection speeds, external CDNs, and syncing with audio, is a lot! Not counting user interactions like seeking, volume, full-screen, etc. Web browsers have a built-in player that would do a lot of this for you now, but if you were Netflix and wanted a really good player you'd probably customize a lot of this.

Granted, React would not be too helpful with the core video player engine, but actual video apps have lots of other features like comments, preview the next videos, behind-the-scenes notes, etc.

And then, if it's two-way video, you have a whole new level of state to track, and you'd definitely need to roll your own to make a good one.