Because this is how the releases are credited on the cover.
I'm going to disagree with the other answers. Unless/until the style guidelines change, I don't think using DE&HO credited as DE makes sense. One isn't another name for the other.
recording artist will often be DE&HO, but that doesn't change how release credits work.
I know, this isn't ideal. It fragments the artist's discography. But that's a problem to solve at the presentation level,
How can the situation be improved then? I think adding an "eponymous" flag to the
member of relationship is the way to go. That would make it possible to display more unified discographies at the presentation level. I'd really like to be able to put that in a picard script.