If Cantonese is as different from Mandarin as French is from Italian (though, IIRC, both are Latin languages), then why are they still called dialects?
I see from the rest of the comments that dialect vs. language is political and arbitrary rather than an objective difference.
If France and Italy weren't different countries, we might well not call French and Italian languages rather than dialects of the same language. The dialect/language distinction isn't particularly clear cut.
Things have changed for Mandarin nowadays. But traditionally: Chinese dialects are only spoken and are regional, whereas Chinese the language (the written symbols) is national -- and very different from the dialects.
The case can be made that Mandarin is now a language and has subsumed Chinese; but the rest are still dialects.