If we look at the past 10,000 years of history - commodities are useful things, objects with utility. Currencies are commodities which people begin to favor for use in trading. Currencies are commodities which have properties that make them good currencies - they have properties such as uniformity, durability, divisibility, portability. This is why precious metals like gold have been favored as currencies through history - gold is uniform, durable, divisible and portable.
The paper dollar bill is, and always has been, completely worthless in and of itself. It is just a piece of paper. Just like Confederate bills were and are pieces of paper, just like the Papiermark was and is a piece of paper. US bills have a history, just as German bills do.
In 1914, the German Mark, or "Goldmark" was redeemable for gold. The Mark/gold link was broken in 1914 and the Mark became the "Papiermark". By 1923, hyperinflation had made the Papiermark near worthless and it was replaced by the Rentenmark and then in the next year this was replaced by the Reichsmark.
Swinging back to the USA, the words "redeemable in gold" were removed from the hundred dollar bill in 1934, and then by 1971 Nixon had closed the gold window entirely. Sort of. Because if he really had, he could have just taken the thousands of tons of gold in Fort Knox and paid down the US debt. But he couldn't. Because that gold still backs the dollar. The dollar was an abstraction of gold before 1934 and 1971, a promise. Then it became an abstraction of the abstraction. It's still on some level backed by the gold in Fort Knox, Trump and the Fed chair could just reopen gold/dollar reconversion if need be, and a hard floor would suddenly appear under the dollar.
Bitcoin has no such floor. There are no thousands of tons of gold waiting in the wings for Bitcoin, no nuclear armed #1 (going on #2) world economy with the power to tax and receive tax payments backing it. We don't even know who created Bitcoin.
It's an odd argument - the only commodity people can find which has no apparent value is the dollar (or euro), so that's what people compare it to.
If this was 1917, you might be asking me why the Papiermark had worth when it was not backed by something, and thus Bitcoins must have value. What people found out was that the Papiermark unhooked from gold did not have value, as the German government of the time didn't give a damn about its value. It's kind of the same argument - Papiermarks in 1917 had value, so Bitcoins must have value. People found out in the 1920s that Papiermarks didn't have value as the implicit promise of government gold conversion went away.