Beastification

The look spread because it worked for the channel that built it, and thousands of creators adopted it wholesale. Whether it transfers is another matter. YouTube's systems have no built-in preference for faces in a thumbnail, so any gain comes from viewers, and a face only carries weight once viewers recognise it. On a channel someone is seeing for the first time, a giant unfamiliar face is a stranger pulling an expression. Beastification also flattens a niche. When every thumbnail in a search result uses the same yellow arrow and the same open mouth, the one that looks different is the one that gets noticed.