In principle, you're right, currency is just traded like anything else; but the fundamentals are different. A company is (presumably) engaged in profitable production (the future profit, i.e. it does something) and is composed of various capital assets (the net value, i.e. it has something). A currency has neither attributes; its claim on future production and assets is only valid in so far as people have confidence in it and will take it in exchange for other things. A share, meanwhile, is a direct claim on things themselves.
Currency can have its own features that make it worthwhile. No matter what happens with btc's popularity, as long as I have one other party to trade with, some features will be available to me through btc that are not available through normally denominated national currencies. Can't I buy btc because I think these are valuable features and their desirability will drive interest and users, just like I would buy Google because the desirability of its features drives interest and users?
A share of an ETF is a claim on a financial asset that is backed by claims in other financial assets, usually stocks. So while it is one level higher than directly owning securities, it is still a claim on productive economic assets.
