There's money involved.
The ethereum itself isn't currency; but it's an asset that people purchase as an investment, which puts it into the same _general_ category as currencies, stocks, bonds, futures contracts, and so on and so forth. Cryptocurrency in general (not just ethereum specifically but also bitcoin and others) is a bit unusual as such assets go, in that it A) has no tangible value except "whatever people will pay for it", like a fiat currency, but B) is not backed by any of the usual sorts of trusted organizations (like central banks) that generally back fiat currencies. This is a somewhat odd combination, but it's not completely unprecedented. There are for example hedge funds that take a net-short position, so that in technical terms they aren't reliably backed by anything really tangible either. It's not advisable to keep your entire net worth tied up in that kind of investment, but as part of a larger portfolio, it can be manageable.
It's important, in contexts like this, to realize that even currency is not precisely the same thing as money. If it were, the government could easily make everyone rich by just printing more currency. That has occasionally been tried, by people who really didn't understand how money works. None of those experiments turned out very well, for reasons that ought to be obvious. Currency is principally important as a proxy for money, and in everyday life when you're going to the store and buying something mundane like a pack of gum, the difference can be largely ignored (provided the currency is reasonably stable). But when you start talking about macroeconomics and investments and stuff, it's important to know the difference.