Because it is invalid. PoS DOES rely on checkpoints for security reasons to prevent a reorg. Bitcoin could remove checkpoints from the code right now and no reorg attack becomes possible. No PoS based system can do so. They would instantly be vulnerable to the "PoS problem" without checkpoints.
You try to avoid my question from above:
Let me restate my question. Why should a node 100000 blocks ahead accept a blockchain re-organisation?