[vitovito]

Because it does someone no good if you have a copy of an old page they're looking for, and they have no way to find you, and you have no way of providing it to them. Centralized services like libraries and archives provide that.

You absolutely should save everything yourself. But you should also give copies to as many centralized services as possible. Lots of copies keeps stuff safe.

[sugarfactory]

"Centralized" was not the word you were looking for. Decentralized systems can serve the searching functionality as proved by many (pure) P2P file-sharing protocols.