For the same reason why the Bible says:
Psalm 82:1- "God has taken his place in the divine council; in the midst of the gods he holds judgment:"
Both the Bible and the Quran subtly acknowledge the existence of multiple gods.
As'salamu Alaikum Everyone,
First, welcome to this blog Awesome31310. May Allah Almighty use this forum to further guide you to Islam. Ameen.
In regards to your question, there are two legitimate answers for it:
1- GOD Almighty can and does refer to Himself as WE and I. So the plural We can and does refer to Allah Almighty Alone.
2- In Allah Almighty's Realm, there are the People of Allah. Even our Hadiths call them AHLU ALLAH
(Family of Allah) 
. إن لله - تعالى - أهلين من الناس، قالوا: يا رسول الله من هم؟ قال: هم أهل القران، أهل الله وخاصته
. This is what we will be called in Heaven, and Allah Almighty also does recognize "Son of GOD" title in the Glorious Quran, but Says that they are no more than pious "....servants raised to honour. (The Noble Quran, 21:26)"
. Nothing Divine about them except that they exist with Allah Almighty Eternally in Heaven. But as far as Divine Authority and Power, all Divine Authority and Power belong to Allah Almighty Alone and none else. You can visit:www.answering-christianity.com/definition_son_of_god.htm (The definition of "Son of God" in Islam)
And here is how you can reach the link for the Hadiths that speak about AHLU ALLAH (the family of Allah) 
- Go to www.Google.com
- Search for إن لله - تعالى - أهلين من الناس، قالوا: يا رسول الله من هم؟ قال: هم أهل القران، أهل الله وخاصته
Iblis (satan) used to be from AHLU ALLAH (Family of Allah in Heaven) 
. But because he and his followers disobeyed GOD Almighty, they were cursed and kicked out of the COMMUNITY OF GOD Almighty:www.answering-christianity.com/adam.htm
So it is quite possible that Allah Almighty uses the WE to refer to his Servants that HE spoke with directly. But even in that case, the WE still also means Allah Almighty Alone (the Majestic We that is), because Allah Almighty is the One and Only Decision Maker
, and there is none other besides Him.
I hope this helps, insha'Allah.