because most people are too lazy to bother with all the hassle of growing their own.  Have to learn how to be a gardener.  Buy the equipment for gardening.  plant it, tend to it, then wait months before you harvest it, then weeks or months again to dry it out.  Bah, more work than making wine with your own grapes.  Just drop by the liquor store, buy a known dose and smoke it within 20 minutes of getting the urge to smoke it.