There are a couple of reasons that the fictitious ancestors show up on Geni. One is that since they exist in early genealogies, which get reprinted and copied, people think they are real. So it is good to have them in the tree, precisely so that the can be labeled fictitious. That way the false lineage can be dealt with directly.
And another is that genealogy is certainly a matter of source work, and lately DNA, but it has a long long history. These fictitious profiles are part of the history of genealogy itself.
So they are in the tree. And they are labeled clearly, so that you can see the layers of history in the tree.
And often, though not always, they will have notes explaining where they came from.
If you follow some of the Welsh lines on up, you can see where they connect to King Arthur. This is because many of the early medieval Welsh Arthurian texts included genealogies. Which of course then got copied, as mentioned above.
Recently I went in and made the Arthurian lines clear, as to how the connections work, and where they appear.
Here is a link that shows that: Caradog ap Bran, King of Siluria
If you look at the profile in tree view, you can see that Caradog's father is a mythical being. There is little evidence for Caradog, but he has children connected to him who actually existed. And children who did not; the line going to Uther, King Arthur's father, is fictional.
So we keep these things in, and we continue to explain them. If they weren't there, we would have to explain why they weren't. Either way, they are an interesting problem.
And part of genealogical history..