The short answer is "because it seems too difficult to have been independently developed." Of course, it was once thought that things like writing or farming we

The short answer is "because it seems too difficult to have been independently developed." Of course, it was once thought that things like writing or farming were too difficult to have been independently developed, but archaeological evidence has made clear at least 3 (and possibly ~6-ish) independent inventions of the former and at least 9 for the latter. There are a few more concrete reasons to specifically believe against independent development of language. If you believe that language faculties arise very quickly after evolution of a biological organ to facilitate language, that suggests that the ability to have language is a key part of Homo sapiens and therefore language would have originated only once in the development in Homo sapiens as a species.