[ppog]

It's not that anyone believes "shelf space is running out." It's signal to noise -- or, to be less judgmental, "findability." Sure, SO or Wikipedia have the storage capacity for people to post anything they wanted anywhere on the site, but that would make much more work for users to find the content they need -- they'd have to trawl through a much bigger mass of semi-relevant or irrelevant stuff.

This is not to say SO or Wikipedia has the balance right; but it's a sensible reason to "limit their users' contributions."