Why ask why? :-)
Again, I come from a religious background that claims Christianity (but most here would dispute that.) There are many differences between mormonism and Christianity, from the tone of voice, style of prayer and concept of God.
Now as a Christian, when I attend different denominations or listen to different Christian teachers, regarldess of creed, the message is very similar. In fact most times I can't tell the difference in teachings between an evangelical minister, or a baptist preacher, or a lutheran priest. If I close my eyes I could be in about any Christian Church. (Catholic Churches are different in this regard.)
If you don't agree with me, try it. Listen to a recorded teaching and tell me if you can identify the teacher's denomination. I'll bet you can't nine times out of ten. There just aren't that many differences in doctrine....most of the differences are in style and are inconsequential IMHO.
I respectfully disagree with you. I can certainly tell a Pentecostal preacher from a Baptist preacher. One will start speaking gibberish, the other won't.
I can usually tell a Presbyterian from a Methodist and I can tell a Lutheran from a Quaker. There are differences in doctrines and to say that there isn't would be closing your eyes to the obvious.
Do Protestant denominations have commonalities? Of course they do, but they all aren't exactly the same. Do you start speaking in tongues when you pray? Do you believe in free will or do you believe in predestination? Do you worship the Sabbath on Sunday or do you worship on Saturday? What is your opinion on Holy Communion? Do you share the same opinion as Lutherans and Anglicans on it? Do you believe in being "slain in the spirit?" Do you believe in the word of faith movement, otherwise known as name it and claim it? Do you believe in church music or believe in no church music? Do you believe in faith and works, or do you believe in faith alone? Do you believe in once saved, always saved or do you believe that it's possible to lose your salvation? Do you believe in baptizing infants like some Protestant churches or do you believe that should come later? Do you believe in women preachers or don't accept that women should be preachers?
Sorry for the long list of questions, but it's an example of where you do hold different doctrines and you don't all believe the same.