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Brokers have only been required to properly track basis for shares for about ten years at this point. So there's still a lot of non-covered shares with not necessarily well kept basis.

There's also things like houses and land that records are likely to be iffy for. Especially because some improvements add to the basis, and who knows where grandma kept her files for improvements or if there's enough detail in there to say for sure.

It's definitely a loophole though, same with donating apprechiated capital property.