Riding on gravel can take a serious toll on your body. You have to work harder to pedal through it, and your bike is probably heavier than a dedicated go-fast road bike. Even on flat-ish rail trails, you can’t really stop pedaling because you will almost immediately come to a stop.
There just aren’t that many nice vintage bikes left out there. Thousands of them were turned into single speeds and fixies between 2005 and 2010. Braze-ons were ground off, frames were spray-painted, and many were crashed and destroyed. Finding a nice one nowadays is rare, and the odds of finding one that’s barely been ridden is even more rare.
When I was a kid, you could ride on pavement to the outskirts of town and then go ride up into the forest, hit a few trails, and bomb a fire road back into town… and you could do all of this on the same bike… and that’s all that gravel bikes are. Riding gravel isn’t new, and technically, neither are gravel bikes.