You could only later get the thing if you grabbed its hash while it was still available. And you could only reliably resolve that hash later if somebody (maybe

You could only later get the thing if you grabbed its hash while it was still available. And you could only reliably resolve that hash later if somebody (maybe you) went out of their way to pin the underlying data. Otherwise nodes would forget rather quickly, because why bother keep around unreferenced bits? It's the same functionality you get with permalinks and sites like archive.org--forgotten unless explicitly remembered by anybody, dynamic unless explicitly a permalink. It's just built into the protocol rather than a feature to be inconsistently implemented over and over by many separate parties.