Garriott may have been harsh in declaration that there are few good designers, he did bring up a valid point. Most who want to and are successful in breaking into the industry do so with training that makes them better suited to roles such as programming, writing, or art design. However, being good at part of the design process does not mean you have the necessary training and skill to be a good overall designer, and like Garriott stated, there are too few options for one to study and train to become an actual designer as opposed to a programmer, or artist, or etc.
Granted, this is not taking into account several outside factors like team-size, demands from publishers, and the like.