Other mistake: McKay and Jackson enters a room and runs into 2 armored figures, Jackson eventually reveals that they are empty suits, however in every shot shot from behind the suits, the suits are constantly moving, as if they are hanging on strings.
Other mistake: In Shepard's apartment, there is a fresh pineapple and other fruit in his kitchen. This despite him having been in Atlantis for several months. This later turns out to be an artificial reality, but that should've been a big hint.
Answer: Because even all 4 Naquadah generators (one was destroyed) together is only enough to dial Earth for approximately 1.2 seconds (As seen in the episode "Letter from Pegasus"). Further, if Earth had found a ZPM, they would dial Atlantis and let them know. After all, the SGC has no other use for a ZPM than to dial Atlantis.
Grumpy Scot