That stories are given a default cover at all should be appreciated. It would make much more sense to not have one at all, and simply have an empty image or a question mark, yet someone has made the effort to create the cover to bring a little more life to the website. Especially when if you want to have a cover, you are meant to procure it yourself.
To ask for more covers, is essentially to ask for a free commission. Someone has to make those covers. Only having a single cover, makes a clear line between stories where the author has bothered to get their own cover, and those that haven't.
Lets face it, the cover is there for one purpose only - to grab peoples' attention. It's not exactly hard to get a basic cover, all you have to do is open up paint, crop the size and write the title and fill in the background. Done in less than a few minutes, and your story will already stand out a lot more.