>I’m so confused that anyone would get hung up enough about this to let it block a migration of this size.

You just said why. A migration of this size needs to make sure "nothing happened in the meantime". If it can't, then any issues or lost record or other kinds of data for accountability are brought into question.

>If we care so much about that it actually looks the same after all that time, why is it not a pdf?

yet another proprietary format?

And honestly, the true answer is probably much more diappointing than any theory I can throw out. Legacy code is rarely about "what's the best approach for the long term?", so I can only imagine legacy documents being much worse about this.