Legibility at 168 pixels wide
Most thumbnails are designed at full size and consumed at a fraction of it. The presets here keep the text large, high-contrast and clear of the duration badge in the lower right.
Fewer words
Three to five words is the working limit. The title beneath the thumbnail carries the detail; the thumbnail carries the hook.
Consistency is the point
A recognisable treatment across a channel does more for click-through than any single clever thumbnail, which is why the presets exist rather than a blank canvas.