Because humans love faces. We love looking at them, we love seeing them. Our brains are wired to handle facial recognition very well and be drawn to faces.
In fact, the effect is so strong that we often see faces where none exist
People being drawn to faces has led to thumbnails with faces on getting far more attention. Tbh, if you were trying to make a YT channel your job, it’s very likely you’d start doing it too, you’d be actively sabotaging yourself for the sake of pride otherwise.