I could also easily carry more than $10k across a border. It's illegal without reporting it (exact number varies). But that's one type of payment.

I've carried $2k USD in just $20s in my pocket across international borders, and had it been in $100s that would be the limit. But if I want to transfer even one million dollars that 100 trips. Or 100 pockets if I ignore the law. I own cargo pants, but even they don't have 100 pockets. And a billion? That's a thousand people each with a hundred pockets full of cash.

What I was saying is that mere logistics makes it hard to move a billion in cash.

So there's a natural prevention of moving large amounts of cash. It's not impossible, but "making it hard" was done on purpose. So cryptocurrencies are not "clever" for "fixing a problem". They're actually removing the amelioration to the problem.