they weren’t, exactly.

neither the pyramids nor the coliseum in rome was ever truly lost to be rediscovered.

The Coliseum wasn’t even necesarily ‘abondoned’, but had rather been repurposed for workshops and housing after it was damaged in earthquakes, and in any case there just wasn’t the interest in the games there used to be. It costs money and resources to keep things up. especially old things, and the people who owned it found that, keeping the games going simply weren’t worth it.

For they pyramids… they were only ‘rediscovered’ by western people. Keep in mind, they were tombs, massive, expensive-to-maintain tombs. for long-dead rulers. Nobody went inside them becuase they’re tombs, and in any case, nobody kept them up because, again, it costs money and resources to do that. And as for exploring their chambers… it takes a certain kind of arrogance to do that, too.

In other places, like the pyramids in south america, they’re lost because the civilization that built them died out, and the jungle reclaimed that land, hiding them. (mayan temples, for example.) Others were, similar to the egyptian pyramids, never actually lost to the culture that built it.

You may notice a trend here. These places are old, and take money, resources and effort to maintain. When times are hard, no one is spending it on upkeeping something that just sits there. not unless there’s a very important reason to do so.