You obviously don't know many marijuana users than do you? A good percentage grow their own.
Let's look at one reason. The cost of purchasing the "drug".
Let's say you smoke one joint a day. A 30 dollar bag will get you six joints or so. so after 6 days you have spent 30, after 12 you have spent 60, after 18 you have spent 90, after 24 you have spent 120, and after 1 month you have spent 150.
150 dollars a month? That's 1800 dollars a year. Growing your own costs far less.