Why should we not pray or worship graven images?

God hates idolatry and we have a direct command from God the Father not to worship graven images.

Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.

Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;

Exodus 20:4-5

And it is also revealed to us that God is a Spirit (so is Jesus Christ and the Holy Spirit) and we should worship Him not with graven images but Spiritually knowing by Faith that He hears us when we pray for He is an omni-present God.

God is a Spirit: and they that worship him must worship him in spirit and in truth.

John 4:24