So why not apply the same logic to other plants? Why isn't everyone's entire vegetable garden grown hydroponically in their basements?
If tomatoes were to cost $20 per tomato, and you could increase your yield and lower your pest damage by growing indoors, you bet your sweet ass we'd all be growing hydroponic tomatoes indoors. And if tomatoes cost $20 per tomato, how long do you think your tomato plants would go unmolested by neighborhood kids?
There's also the issue of outdoor grows being perceived as a nuisance by neighbors who don't agree with MJ use. My neighbor has a nice outdoor grow in his backyard that he keeps out of sight, but once during a heavy rain he moved the plants onto his porch. The rest of the street were all talking about it, some in not-nice tones. He's a senior who uses the plants for pain relief, but it's still a huge stigma. With indoor grows you avoid all of these problems.
And yes, there are probably some people who are growing more than their share of plants (but remember - they can have up to X plants PER CARD, so if they have a few friends with cards who are not growing, they can pool them to do a grow together up to a certain amount)*. But sure, go ahead thinking that indoor growers are all asshole criminals skirting the law if that's what you want to believe.
*[In Colorado, home growers are allowed to have up to six plants per person or 12 per household, but you can only have half flowering at a time. When you grow outdoors, expect all plants to flower at the same time.] - another reason to not grow outside