Because that doesn't attract viewers like nudity does. How many times have you seen a half naked chick as the thumbnail, yet never appears in the video?
Remember when YT immediately counted every view regardless of how long someone watched, and there was this plague of clickbait? For example headless women showing their cleavage in the thumbnail, because there were enough horny idiots dumb enough to click the thumbnail.
That's why they switched to 30 seconds initially, and now to something more percentage based i think
(Funny enough it is practically impossible to Google the articles that described these changes when they first happened; I read them bs k then and I've been looking for them for ten minutes to no avail)
I honestly wouldn't know - I don't usually watch videos with half-naked chicks in the thumbnails. Basically, when I'm watching on YouTube and not directed there from another video site, I only watch Takeshi's Castle, Cinemassacre, Channel Awesome, TheStrawHatNO!, newfiebangaa, World of Longplays, Playable Passion (sensing a theme?), Guru Larry, and a few other channels along that same theme.
