Moisture control is a huge part of any housing envelope's job. Vapor barriers and thermal breaks (because any temperature change is also a relative humidity change) can make or break a home design, and plenty of attractive concepts fall flat when the seasons change and suddenly there's a thermal gradient leading to condensation in the wall that provides an ideal growth medium for microbes. Moist paper = food.

Fire resistance is another biggie, and cardboard isn't good at that either. By the time you've treated it with fire-retardants, you've drenched the whole area in PFAs and could've just used sheetrock or something that's inherently inert.