Back to the original question asked by the OP:
Why does everyone assume Moss makes Brady great?
Who exactly is this 'everyone' that you are referring to?
The only places I can recall seeing that type of talk is on messageboards from fans of other teams, typically after an article about Brady.
They're fans of other teams, so any time something is written about a member of the Patriot organization, they disagree with it and try to find a way to refute it. If it's a positive article about Belichick, they'll say it was all luck for drafting Brady. If it's a positive article about Brady, they'll say it was all Moss. If it's a positive article about Moss, they say he's a slouch who never won a championship. If it's a positive article about Kraft, they'll say he's cheap and is nothing without Belichick. And round and round the circle of excuses go. There is zero substance and value to be found in those comment sections.
If I'm wrong with my assumption on where you got your idea for this thread, I apologize. But if I'm correct, then my best suggestion I can offer you is to always avoid the comments section of any article from national sites such as ESPN, CBS Sports, FOX Sports, Pro Football Talk, NFL.com, etc.