Because before you many users complained "IT TAKES UP THE WHOLE SCREEN!!!!" and it was a bit annoying to be honest when it obscures a video or something else you're trying to view.
But it doesn’t take up the whole screen, it flashes for like 2 seconds, and it has been around so long that it has created a very ingrained user behavior.
I’d be very curious to see how many complaints they’ve actually gotten about it. This definitely struck me as “random”
what kind of video are you watching where you need to change the volume so often and missing two seconds of part of the video would be such an issue?