While I can understand that when a video is fullscreen the media controls are hidden to offer the entire viewport to the video and create an immersive experience, I've been curious about when the video is not fullscreen.
When a YouTube video is embedded in a webpage, the media controls are only shown when the video is paused or currently being hovered over. I don't believe this is purely to create an immersive experience, as there is other (potentially distracting) content around, such as related videos, comments and chat, etc.
Surely this is contributes towards a bad experience, as it hides the most commonly used controls on the interface.
Other video streaming services also seem to do the same thing, although a lot of desktop video software (such as VLC) seem to keep the controls on-screen by default.
The only possible reason for this I can see is to hide the progress bar so the viewer can't see how far along in the video they are, meaning that they can be more immersed in it regardless of other elements on the screen. Is there anything else this behaviour contributes to?