American currency isn't paper. It's more akin to cloth than anything else. And you can go swimming with 300 in bills in the USA. They are not going to melt and will be just fine even if submerged for hours.
There are absolutely better materials to use, no argument there. But US paper currency are not even close to being fragile.
It's paper. It just happens to be a cotton/linen long-fibre pulp blend rather than, say, wood pulp. That's how most paper was made from the origins of the stuff, in the West at least; wood pulp and so forth are relatively new in the grand scheme of things.
There are absolutely better materials to use, no argument there. But US paper currency are not even close to being fragile.