They aren't, as far as I can tell (my wife is a successful visual designer). They tend to be started by individual designers who find a successful niche, or (less often) design collectives that band together to hire business staff.
That's not the point though. The bulk of these people are getting majors they never use. I would argue they should never have been pointed in the direction of an industry they don't have the aptitude for, no matter what their 'dreams' are.
