Because the Hebrew word "Elohim" which is the word used for "God" in the Bible is plural, meaning "gods." The word also has both masculine and feminine attributes. It is mistranslated into English as singular and as "God." This is why this occurs throughout the Bible. The reason for this discrepency is under debate.
The first theory as to why "God" is plural in the Bible is that of most religious conservatives: that it denotes authority and sovereignty, much as royalty would say "We" and "Our." Most religious conservatives frown at the thought of polytheism existing in the Bible and this theory brings them comfort.
Most researchers and well informed scholars believe that the Abrahamic Faith began as polytheism, which later took on a more monotheistic form. According to this theory, the God of the Bible began as a High God in an ancient Semetic pantheon of Gods. From this YHVH became more of a tribal Deity of Israel, and over time He became the "only" God. This is supported by reading the Holy Writ and how the concept of God evolved over time. In the Book of Genesis God is neither omniscient or omnipresent, and He even wrestles with Jacob all night and loses! Later God becomes more remote with the later Books of Moses, but is clearly shown as the Israelite God, not the only deity in existence; but that the Jewish God was stronger and could beat up the pagan deities. Then later in the Psalms He is shown as the Only God. This change over time seems to support that prior to this the Jewish Faith may have been polytheistic in nature, and thus the Hebrew word would have been plural.
The last theory is utterly incorrect. This is the one used by Christians that know nothing of religious history. These people claim that it refers to the Trinity. This is absurd. The Jews had no Trinity. God is Achad, "One." In fact, the concept of the Trinity did not even exist until the 4th Century with the bishop Athanasius and, until then, was an alien concept even in the Christian Faith.
The reason for the plurality is the nature of the Hebrew word for "God," which is plural and may refer to a pantheon collectivley or to the angels which are the deity's vicegerents on Earth. The reasons for the use of "gods" instead of "God" is debatable. the three most common theories are listed above, the latter of which is the result of stupidity. Only you can judge for yourself as to why this occurs.