It worked. Then someone wrote about it, and everyone did it. Then it became a negative signal. There seems to be a 5-10 year trend lag time on stuff like that where the least self-aware self-promoters don't realize it doesn't work. Eventually a new trend falls to their level, and the cycle begins anew.

A big reason everyone has the big, irrelevant image on top is that medium was designed to work best with that assumption. If you actually blog there, your index won't look good unless you have a header photo for each post.