Why aren't HVAC ducts made out of different materials?

Hey guys, newbie here.

I've been learning about HVAC systems the last couple of days for fun (yay unemployment!) and I've always been curious about how (poorly) HVAC ducts are made and it hasn't really been addressed adequately.

Is it me, or are they really poorly designed?

Why are they made of either metal or flex? The flex is easily damaged, doesn't last long, porous on the inside and difficult or not safe to clean. The metal doesn't hold temperature and is not air tight (held together by tape? Correct me if I'm wrong)

Wouldn't a large diameter PVC or some kind of plastic or hard rubber flexible tubing work so much better? Or something better that I haven't thought of? Does it simply come down to cost? What are the million dollar homes or families with special needs or allergy sufferers doing? It's hard to believe that there's not a 'top of the line' better product above sheet metal? I can't be the only one thinking this.

As I said, I'm a newbie and just now learning about HVAC systems and how they work, so go easy on me if this seems like a dumb question.

Thanks!

submitted by /u/jodermacho
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