If only people who had downloaded the torrents would upload thumbs made from the download. Even the original seeder so often doesn't post extra pictures.
I often add one thumbnail picture to a torrent which only carries one cover picture. It will come from a file I already have on my hard disks. I'm not taking too much trouble. But I think it might help downloaders knowing what they might get. No, don't ask me why.
Every downloader could (COULD) add some pictures to most torrent pages but doesn't. And passes up the chance of a point or two.
I actually don't see many bad uploads of pictures. Some I report if they are really misleading.
And the link you give is to an original post in 2010 - so no-one seemed much bothered until now