Because it's slightly confusing. It only shows a play icon when to thumb shown is of a video.
If there's a video in the album it doesn't show the icon.
The alternative: showing a video icon on any thumb if there's a video in an album would be confusing. Because the video icon could be on a thumb of a simple image.
So let's just not show a video icon on a thumb in the gallery overview.
To make sure: do show the thumb within a gallery on a thumbnail if it's a video.
Because it's slightly confusing. It only shows a play icon when to thumb shown is of a video. If there's a video in the album it doesn't show the icon. The alternative: showing a video icon on any thumb if there's a video in an album would be confusing. Because the video icon could be on a thumb of a simple image.
So let's just not show a video icon on a thumb in the gallery overview. To make sure: do show the thumb within a gallery on a thumbnail if it's a video.