If Norwegian, Swedish, and Danish are considered separate languages, or Serbian and Croatian, then I can't see how Scots is not a separate language from English. I guess a lot of it comes down to politics.
On the opposite side, consider the many "dialects" of Chinese: Mandarin, Cantonese, Shanghainese, Taiwanese, etc. Someone who only understands one of these will be pretty much entirely unable to understand the others. Yet we give the Chinese a nod and let them consider all these different languages to be just dialects.