History has always gotten rewritten throughout time. If you have a giant library it's easier for bad actors to gain influence and alter certain books, or remove them. This isn't just theoretical, under external pressure IA has already removed sites from its archive for copyright and political reasons.
There are also threats that are generally not even considered because they happen with rare frequency, but when they happen they're devastating. The library of Alexandria was burned by Julius Caesar during a war. Likewise, if all your servers are in one country that geographic risk, they can get destroyed in the event of a war or such. No one expects this to happen today in the US, but archives should be robust long term, for decades, ideally even centuries.
We are talking about an (almost) worldwide archive after all.
I would wager at least 95% of "digital memory" archived is just absolute garbage from SEO spam to just some small websites holding no actual value.
The true digital memory of the world is almost entirely behind the walls of reddit, twitter, facebook, and very few other sites. The internet landscape has changed massively from the 90s and 2000s.