What was callously and deliberately left as Ground Zero is far from "unretrievable."
The remains found at the WTC site are most likely only a small percentage of those that were left at Fresh Kills.
Despite the relatively fine sifting that was performed there, remains had to survive being run over by bulldozers and compacted into the landfill in order to reach the sifting.
Fields of debris were dumped and spread, workers would then walk through the fields, with time to make only one pass before the debris was pushed into a pile and a new load dumped. Unfortunately, the ground was very wet and muddy from continual watering to keep the dust down.
When the next load was spread out, any remains left over from the previous load were compacted into the mud. It was frustrating to have seen remains but were unable to pick them up because your hands were full, and the debris was scooped up before you could return from the temporary morgue.
I can't speak for those who managed the project, and don't know if they were aware that the pace of deliveries resulted in remains being left behind, but nobody with whom I worked in those debris fields ever callously, or deliberately left anything behind.
And yes, it was deliberate.
And I am not saying the workers deliberately did that. It was a senior management decision.