It's because WP is an encyclopedia, not a bulletin board.

If people aren't literate enough to understand text well-enough to restate it in their own words, do you really want them adding that text to WP? It may be obviously wrong, it may be misleading, it may be off-topic or not fit logically into the surrounding text, etc. A copy-pasted "encyclopedia" would be an ugly mess. I frequently find a sentence or paragraph just tipped in somewhere that's in the wrong place. That's a collage, not an article.

It's a problem for WP because people are supposed to be able to re-distribute it under the right licence, and slapping copyright restricted stuff prevents that happening.

I'd image if it's not copyright infringement on the one article across the entire Wikipedia it could become copyright infringement.