I mean, in hindsight it actually shouldn't be surprising.
People across the ages would of seeked various ways to live away from the Grimm and terrain is one of the main ways.
Not to mention Solitas became known for being a pretty good spot for finding veins of Dust.
So a terrain that at one time made it hard for Grimm to chase you, and a source of a defensive resource, and boom, a hotspot to migrate to.
And look at history in our world and you'll find accounts of various groups calling the same areas home over time. Following resources, legends, or looking for passage. I mean, those ruins in the image above is literally just Stonehedge. And the real life one and sites like it to this day have researchers and archeologists scratching their heads.