If this is a stupid question, you can tell me, but also please tell me why. I've done a little research and can't find a good answer, why don't they make PVC duct-work for HVAC?

Seems it would be much easier (more DIY friendly and faster for the pros) and possibly less expensive than using metal. Also, it would be easier to seal the duct system. We use pvc for water, and venting/exhaust, but wouldn't large pvc duct-work, either round or rectangular glue together panels also work?

Oh ya, if they do make it, no one in my area knows about it.