The only confirmed cases of Giardia that I have ever encountered were in Morocco. I have certainly encountered a lot of folks who claim to have had Giardia but have never had it diagnosed. Interestingly, I had an Alaskan Malamute who contracted a diagnosed case of Giardia after a stay in a boarding kennel in Seattle.

Until recently, I never treated my drinking water while hiking and climbing in the U.S. Aside from the occasional case of the "green-apple-quickstep", I have never suffered any serious GI disease "at home". I'm getting a bit long in the tooth now and don't have the resistance to infection I once had so I've started treating my water if it looks suspicious.