Not sure that's *quite* true. I've never grown the stuff myself, but I know folks who have, and while it's easy to grow, it's hard to grow stuff with decent quality, and takes a fair investment of time and money. You don't get a high THC content from something growing in a ditch by the interstate.
Just like there's millions of people who make their own beer and wine, but most of us prefer commercially-made beer and wine to, say, uncle Don's home-brewed pilsner and apricot chardonnay, there'd totally be a market for commercially-grown weed with a consist THC content.
I've heard various theories about why weed was made illegal. Regardless of the root causes, one of the reasons it stays illegal is the massive investment in law enforcement infrastructure. People make their careers and reputations from the Drug War. And there was the failure of Proposition 19 in Cali, which was defeated in part due to votes from cannabis growers and sellers, who would see their prices drop and taxes rise if their product was legalized.
If you haven't seen it, I suggest the movie "Traffic" by Stephen Soderbergh (2000), that gets into what I'm saying here. And it's a great movie.