NOt sure i want to do this because I amnot in the mood to arguethis with people anmore, but you asked the questions so i will answer, yes it is forbiden in the bible.
The Bible strongly warns people not to consult mediums and spiritists for the truth, but to inquire of God (Isaiah 8:19). Clearly, if those who practiced these arts have any power (beyond being great deceivers), it is not a gift from God (as some falsely claim). The Bible condemns and forbids these practices several times (Deuteronomy 18:9-14; Isaiah 44:25; Jeremiah 27:9; 2 Kings 21:6; 23:24). Divination and Spiritism were despised practices of the heathens (Ezekiel 21:21; Isaiah 19:3; 1 Samuel 28). Scripture says that one of the reasons King Saul died was "because he consulted a medium for guidance" rather than God (1 Chronicles 10:13-14).
The Bible provides examples showing the folly and failures of those who claimed the ability to predict the future based on their own powers or those of spirits (Daniel 2:27, etc.). God calls the word of diviners "nonsense," "lies" and "deception" for foolish people (Ezekiel 13:8; Jeremiah 14:14; Isaiah 44:25). God said, "Woe to the foolish prophets who follow their own spirit and have seen nothing!" (Ezekiel 13:3). If an Israelite lived the deceitful practice of being a medium, it was punishable by death (Leviticus 20:27). The presence of a medium or spiritist among God's people of Israel was considered a defilement (Leviticus 19:31). All these practices take people further from their Creator, the true and living God.
It further more goes on to say, that the act of communing with spirits is the act of witchcraft, and theirfore is condemend as an act of evil.
As to why they put it that we can only guess, it could have been that as christians grew in the pagen run empires of the times, they wish to condem them as evil so that they would be justified in their acts against them, it could have come from their god as well. this we will never really know as it is a matter of beleif. If you do not beleive in the bible being real as I don't you take reason one if you do then you take reason two.