Can one use large-diameter rigid plastic pipe for HVAC ducting slash why is this never done?
1)  cost.  plastic is way more than steel

2)  weight.  so much heavier per unit volume

3)  thermal expansion and contraction, deformation, etc.

4)  it is used in lab environments for fume extraction, and in exhaust for high efficiency gas appliance exhaust (for its resistance to corrosion from the combustion gases)

5)  you also cannot use rigid plastic ducting for interior air distribution for fire spread reasons. the fire code forbids it.  imagine a flamethrower coming out of your vents as a small spark is fanned to superheated air as the forced air fans the flames from the burning plastic.