Ever page added loses something. Can you imagine how absolutely useless an infinitely long book would be? Heck, how many people would read a 5000 page DMG? Or even bother to reference it? It needs to be long enough to cover the subject, and short enough to be interesting.
Repetition is important in learning a new subject. Now, with a book one can re-read it for repetition, but that assumes it is written in "the right" way the first time. Which can only be true for a subset of readers. A well written instructional manual will cover difficult topics several times, in different ways and with different approaches or examples each time. Because people are different.
There are so many resources and ways to learn to DM today that anyone who really is interested should not have an external challenge in learning how to do so. Books, videos, blogs.. tens or hundreds of thousands of them.
None of that is to say the DMG could not have been better written or edited.