The question isn't why wouldn't it or why couldn't it, but why must it.

Many of our maps could equal the territory, or at least match it precisely. For me, the territories that my present understanding of math maps to are:

1. physical reality,
2. what happens when others and my past and future selves "do math".

I expect a proof of a theorem in my mind right now to tell me that

1. physical reality will behave a certain way if the theorem applies to my model of it, and
2. when I or others "do math", we'll have ideas that are either consistent with the theorem, or ideas that have a particular reflective feeling about them that I've learned to call "errors".

I adhere to this belief --- this trust in math --- more faithfully than to any other I can think of, and have seen more evidence for it than, well, almost anything. But I'm still not sure that it must "equal the territory", and I think that's the point.