Mostly because you've entered into an agreement with the first party (however dubious, that's another argument) and have consented to their terms to use their platform. There wasn't a checkbox that said "I consent to third parties storing all of my information after we decide to shutdown the platform".

Consent is paramount to privacy, and archiving everything without consent is a breaking a huge privacy and trust boundary.

Going into this, we did understand that this could be a concern. We are committed to working with group moderators as we work to make what we've saved available, and will be honoring valid take-down requests for content that people that prefer the content to not be online.

Sorry, but you're doing consent wrong. Consent is something received before an action.

Would you mind clarifying your objection for me? Don't authors of posts assume the risk that third parties will copy and reuse the content when they publish it?

Just because the risk is there doesn't mean we should be forced to accept it. There is currently a battle waging for our privacy.