Because we don't know enough about language diversity. Until recently, most linguistics probably took for granted that all languages had recursive traits. The recent controversy over the Piraha language is now calling Universal Grammar into question. These are assumptions about how the human brain works that cannot be challenged without investigating real languages. An outsider can't document a whole language in the time it now takes for it to disappear completely, and we can't understand the fundamental nature of human language with the meager dataset we have managed to fully document.