The biggest reason why we don't do this is because it wouldn't provide a consistent/accurate timeline view of the buffered data.
If we just restarted the player (how it acts when you hit menu and go back to the channel) without warning, we could potentially lose a buffer that someone was still watching.
Because there's a lot of details around getting this right, and for most of the use cases of our customers this is not an issue, it has not been a high priority for us to tackle yet. It's still on our radar to make it better, but we aren't sure when.