For the rather obvious reason that mining isn't a sure way to make money (bitcoin price may crash, difficulty could increase dramatically, other unforeseen events may happen), while selling miners is a sure thing. In a gold rush, it's not the miners who get rich, but the guys selling shovels.
That's what I always say, and I am trying to think of a new "shovel" to sell, but the amount of money they would make is ridiculous and they could have earned back their investment tenfold by now