Labo goes agains the consencus that children's toys need to be somewhat durable to stand against theim for a good amount of time. Paper is not that durable.
Cardboard is also extremely cheap and replacable. You can make your own creations from nothing but boxes what you'd throw out for recycling. It is an innately brilliant idea. Children destroy things. Labo makes total sense and wouldn't work at all as an introduction to engineering if it was made of preset plastic.