My guess is that he's just not getting offers (many offers) to release new material.
MC disagree. Many people are waiting on projects which have been discussed ad nauseam on these boards and there's heaps of people willing to work with DOOM. My theory is that he's a perfectionist, that stuff isn't going to be released until it's just right. Even the projects with Madlib and Jneiro took years.
Whatever people say, the stuff he releases is damn well produced, and like you said he ain't making mixtapes and may not have put out a full length record for 18 months but still new collab tracks are dropping now and then for those of us who jones.