Cap gains on investable assets may not be that hard if you can find the records of when shares were bought. But for some those records might not exist? Some brokers did not track basis until recently.
On real property - establishing basis is a lot harder. Dad did not document the value of the property he inherited from my mother at her death, although he had the property resurveyed. I guess he didn’t have an accountant to tell him to do that, and the lawyer didn’t mention it. I always assumed that had been documented when Dad inherited, but the lawyer said not with him mentioning an accountant would have done it, and Dad doesn’t remember such a thing and didn’t have an accountant other than the lady that does his taxes. Maybe because of spousal inheritance rules basis documentation was not necessary. Just to be clear if someone is confused, the property was inherited by my mom, and inherited by my dad on her death. It was not joint.
But he’s also made considerable home improvements since, and the documentation is haphazard. There are receipts for some of the materials, but probably little for. I do have boxes of records. But he inherited happened in 1995!
The house itself is not valued very highly by the county. The farm land however is worth a lot.
Some, probably most people just don’t bother to track these things while alive, not thinking or being aware what is needed for their heirs to deal with.