Paper has a whole lot of chalk and sand in it so is an abrasive. Cardboard even more so with all sorts of recycled materials in it including nylon and plastics.
Much of why a knife or machete breaks is to do with the design, metal and heat treatment. Good designs, well made with a good steel correctly heat treated for the intended use rarely fail unless abused, forced to fail. Poor knives have some part poorly done so more likely to fail.
I've bust a good few knives. Half have failed due to poor design. Here stresses and strains were focused in such a way that the blade failed. The other half through poor metallurgy and me doing something that metal wasn't able to cope with. Some failures I can understand because I abused the tool. Others were poor show as the knife should have been up to it.
There is a whole lot of hype and marketing behind the sales of knives. At the end of the day its steel and manufacturing correctly to get the right outcome for a particular task. Some manufacturers get it more right than others. A whole lot comes down to reputation.
There are in my opinion two types of knives that fail in different ways. Rule of thumb rather than absolute:
Soft blades that the edges cave in and hard to keep an edge. Tips can fail through bending fatigue.
Hard knives that snap either forward or back from the point of balance. Tips just snap.
Put enough metal, thick, in a blade and its hard to make them fail if made well. Trouble is they are heavy and don't cut. Very thin blades like kitchen knives cut brilliantly but give them tough thing to deal with and they buckle as there just isn't enough metal there. Whatever is done its always a compromise. When you pick up any knife make a decision on what you think its limitations are, and then they won't generally let you down.
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