Well they clearly are because we just got the artificer. And that's most likely because they realized that their initial draft of pigeonholing it as a wizard subclass was a disservice to the concept.
Note that the Artificer was developed as it was specifically needed for a particular setting.
As others have said - if they are going to do Dark Sun, then they'll need a Psion. But until they do Dark Sun... they don't have a compelling need to do a Psion.
More broadly, I kind of agree with a general point - a large number of classes we could imagine are probably best addressed as sub-classes, off the rather generic bases we already have. There are a few cases where I might agree a stnad-alone class is called for, but that may not be terribly often.