But we're dealing with such small numbers and such perishable materials! This isn't like modern humans - billions of individuals each generating tons of waste

But we're dealing with such small numbers and such perishable materials! This isn't like modern humans - billions of individuals each generating tons of waste materials over their lifetime, stuff like concrete, plastic, glass, and metals. This is a few hundred thousand individuals who are sometimes making things out of sticks, grasses, and hide. As far as I can tell from some quick googling, there only were ever a few hundred thousand homo heidelbergensis, and that's the high estimate. We only have a few hundred specimens of the bones of homo heidelbergensis and some of those are from extremely bizarre circumstances, like a pit in a cavern where people seem to have been deliberately disposed of. If we only have a few bits and pieces of bone left, consider how much less likely it is for a structure of plant and animal materials to survive. If it was in use by people, eventually it would wear out and be replaced, and get broken down to reuse its materials in various ways. If people abandon it, they probably won't bury it like they would a human being (very convenient to future archeologists). Abandoned structures will just stay on the surface, where the elements and decomposing organisms will eventually reduce it to dust. According to the researchers here, the wooden structures only survived due to being preserved in fine sediment in the water. So we're only ever going to find structures like this due to wildly improbable events. It's amazing we've found any!