Make It Free!!!

With Valve making headline after headline releasing their old games for free, one question comes to mind: Why don't more companies do this?  If a company has games just collecting dust, why not allow the masses full access to experience a little bit of what your company can do?  It seems that the one advantage for valve is that it gets more people using their Steam software, but for other companies it would allow a new generation to experience their older games, and maybe even hook them into franchises that are still alive and kicking.