Hollow core doors have masonite skins which does look a bit like compressed cardboard. Both use coarse fibers. The inside has cardboard spacers. Nothing really wrong with this, masonite is a perfectly acceptable material as long as you don't repeatedly expose it to water. The cardboard spacers will still be there 50 years later, the door is sealed. I'm not trying to claim they are anything other than what they are, basic interior doors that serve their purpose. Theer are millions of them out there.
Vinyl and other synthetic composites are becoming more common in construction, particularly in areas that may get wet. It doesn't rot, warp, or shrink. Again there is nothing intrensically wrong with it. Would a good hardwood or cedar be better? Sure? But are composites better that new growth pine or fir? Probably. If you've had to work on a house that had new growth pine for exterior trim combined with not getting painted as soon as it needed it (and how often does that happen) then you'd know what I mean.