Because the Euro and the US Dollar are legal tender in their respective jurisdictions, thus giving them a competitive edge over other types of money, and — perhaps more importantly — because capital gains are calculated using each currency as a reference in the jurisdiction where it’s legal tender.

So, if you invest in Euros in the US, and the dollar falls relative to the Euro, you owe money to the IRS — and vice verse for the Euro in Euro-countries. It’s difficult for competing currencies to gain traction when you owe money if the legal tender drops in value (relative to the competing currency).

Read the OP, then read my comment... Yes, I understand why 2 different currencies have value. The OP does not. Rhetorical question.

Because both currencies have monopoly in two highly developed economies. I guarantee you that the exact moment the government crashes or drops support for the currency, that currency becomes worth zero. Happened many times in history. However, I bet the chance of either US or EU collapsing is tiny.

My comment was rephrasing the OP so they'd understand why they're argument was wrong, not an actual opinion.

Sorry for quick shallow reading on my part.

I've lived and worked in many countries, I know why sovereign currencies exist.

I don't understand your point. I can understand we will have multiple crypto currencies, but if there are unlimited number of currencies, why would any of them have any value?

If businesscards, party flyers, and other pieces of paper exist, why do dollar bills have value?

Not sure if this is rethorical, but it's a great question. In hacker-news-pedantry tradition i will answer:

A. A dollar bill represents an accounting entry in a bank account. Banks are the third party that facilitate trust and the rules by which 2 actors trade. B. Dollar bills are legal tender for the payment of taxes. They are enforced by government, which is the sole legal arbiter of coercive action. Taxation requires every citizen to hold a reserve of state currency. C. Due to the specialized printing process dollar bills are, at least theoretically, hard to counterfeit. D. Ubiquity as trading mechanism, barter requires double coincidence of wants (rare) or interpersonal credit (lack of third party oversight to guarantee fair play). When holding assets, these can deprecate (or appreciate) in value, and would need to be converted in cash holdings to avoid the impracticalities of barter exchange. E. They are an convenient way of turning digital entries into tangible user objects, since they hardly take any space.

I'm sure you could keep the list going. Notice how cryptocurrencies satisfy a lot of those conditions. Also notice how bitcoin in particular fails at a number of them.

Because you need it to pay taxes in European nations?