Because Allah is just a human thought pattern in Muhammed's mind.
Muhammed knew some Jews who though Ezra was God's son...and well! that is in the Quran!! Muhammed knew some heretical Christians who thought Mary was part of the Trinity...and that just made it's way in the Quran. Jesus talking as a baby from the cradle and under a date palm, and making little birds from clay...all of this proves the Quran was influenced by seriously corrupt religions in 7th century Arabia.
Quite ironic considering the Quran is believed to "correct" the other religions.