I ran a game where society had fallen and bounced back, but to nowhere near the level of power it had before. The areas that were civilized didn't have anything right next to them, but some areas had been left generally uninhabitable, and the scavengers had been through places and picked everything clean that they could. So how did I justify some things off the beaten path?
Well, there were some areas where the ruins were hard to cut through or destroy, but would lose cohesion if you did that work. This meant that unlike, say, a big stone pillar, this type of partially magical substance was really unhelpful, and you'd only clear it out if you wanted land. Of course it had some of the worst feral demihumans crawling around it, as it was an area where agriculture wasn't really going to help, so then I had a wildland with interesting enough ruins that weren't worth most people's time. The deus ex machina there was the ruins themselves.
I had another area that had been a wonderful city, and unlike the place I just described, people still lived around there, and most of the annoying pieces of the structure had been carted off. But because the formerly wonderful city was perceived (correctly) to offer even more secrets if explored the right way, it was a place where the nearby nations contested it from time to time, and deployed academians (or paid successful academic kooks who had discovered anything interesting). As such, this was a place where the PCs could go and interact with eccentric people of different academic backgrounds who were all busy trying to understand different details of this place. This was less deus ex machina and more proposing a social structure that I hoped sounded plausible. I'm less sure I succeeded here, but I still like the idea well enough that I don't consider it a failure. Certainly I feel if I had put more effort into it, I could have decreased the mental cost of buy-in.
In general, if you want something that's not too far from civilization and people don't explore it, there's a couple other things to use. First, people could simply be scared of it with good reason- explorers either never make it out, or get injured by traps or monsters that groups either don't find, or disappear completely. Second, you could have a powerful faction that believes it shouldn't be screwed with, for sentiment, religion, or superstition (you actually see this in the real world). Third, gaining entrance beyond the long-stripped outer region of whatever your thing is could be something that is dangerous even to a medium sized army.