Earlier quoted context omitted.
Dollars are also tied to nothing. A towel with the logo of a sports team also has zero intrinsic value. Same with a shiny lump of useless metal. That's because being tied to something, or having intrinsic value is not how things gain value. Things, crypto, dollars, gold, and towels, have value because people WANT them. That's it. You even touched on it: "some sort of service that people want, and therefore have value…
US dollars are backed by the US economy and military
The mining network is a lot more productive than the US military which has vastly negative economic output. At least ASICs doesn't wander around wrecking countries in a semi-random fashion. The mining network even operates at a profit, which is much better than the US government by a huge margin. Someone thinks they are worth the money.