Because spinning in a circle doesn’t smell bad,
Peruse this thread (actually an interesting one), then convince me these people aren’t interested in copping a buzz along with enjoying the finer points of their chosen poison. 
I think the simple answer to the OP (for the US) is that the US government used drug laws to attempt to control immigrant populations and this mentality has carried forward to this day. There is now such a large industry devoted to fighting pot use that I don’t know how it will ever get changed. When you have hundreds of thousands of career law enforcement professionals who’s bread and butter is keeping people from using pot that makes it difficult for the real facts to guide our nations drug policies.