Why do Christians say they have only one God but pray to both Jesus and God?

I don't know.

By making up trinitarian BS they rationalize this nonsense away!

Because they have dismissed Jehovah as the only true God, and have discarded Jesus as a literal "mediator" between God and men. (1 Tim.2:5)  And have redefined Jehovah's spirit (or "power")- which is "holy"- as a 3rd, nameless person of a proposed trinity.

With the Bible, one can debate the point in various ways, from various sides. But what is already a matter of history, is that the Trinity- the concept that permits this plural view of God- was adopted by the newborn Roman Catholic Church, in late 3rd and 4th-century Europe, after being formulated with the help of Greek philosophy.

The Judaic Christians- or "Early-Christians" (both Jew and Gentile)- did not worship a Trinity, nor worship Jesus; not as God, anyway.

This history, is what led Catholic and Protestant teachers to later call the Trinity "new light".

But actually, the Trinity wasn't even "new light" to humans in or before the 4th-century. Historical records are reasonably clear about the fact that the pantheon of gods that stretched back through Rome, Greece, Egypt and Babylon, were headed by popular trinity godheads, whose origin is not traced back to Jehovah, but to human political and spirtisitc unions of philosophy and development.

This is too important a matter to argue over, or maintain animosity between peoples; that would be petty.  Yet, each individual must respectfully make their choice, while keeping in mind a number of realities, including the Bible, and Revelations chapter 17 and 18:

For "No light of a lamp will ever shine in you again, and no voice of a bridegroom and of a bride will ever be heard in you again; for your merchants were the top-ranking men of the earth, and by your spiritistic practices all the nations were misled."- Rev.18:23

Trinities were also popular, during Jesus' time, among the Gentiles.

“Listen, O Israel: Jehovah our God is one Jehovah."- Deut.6:4  

Mark.12:28-29- “Which commandment is first* of all?” (And) Jesus answered: “The first is, ‘Hear, O Israel, Jehovah our God is one Jehovah."

"Jehovah your God will cleanse your heart and the heart of your offspring, so that you will love Jehovah your God with all your heart and all your soul and you may live."- Deut.30:6

Of course, many popular contemporary Bible translations, have now removed God's name from the divine text, which does little to help identify Jehovah as God.  So one will have to decide for themselves, what sounds accurate, and what does not.

I'll give you the Catholic "take" (How we look at it.)

One God, one nature (divine), three persons (through Generation and Spiration)---Father, Word, and Holy Spirit.  (John, 1: In the beginning was the Word, and the Word was with God, and the Word was God.)

Second person, the Word, became man, Jesus Christ.  Jesus had both a divine and a human nature.  (Hypostatic Union---is a technical term in Christian theology employed in mainstream Christology to describe the union of Christ's humanity and divinity in one hypostasis, or individual existence.)

And if you get to the point where you really want to try to understand this, FJ Sheed has a method in his book Theology and Sanity that will give you a good shot at making some progress in understanding.

A complete explanation may not come until you are face to face with Him in our subsequent existence in what is usually described as "Heaven."

Make sense?....  Personally, it does to me.

And let me also throw in two quotes with this edit:

Jesus told us to pray to the Father in Matt. 6:9 when He said, "Pray, then, in this way: 'Our Father who is in heaven, holy Is Your name."

Jesus said in John 14:14, "If you ask Me anything in My name, I will do it."

For myself, whenever I get any kind of gift I always try to personally thank everyone who was involved in the giving.

The problem that existed in the 4th century, within the Christian religion, was many divisions. Some claimed Jesus was not actually a human, some claimed Jesus was not God, some claimed Jesus a different being sent by a different God then the Hebrew God, or even that he was not from a spiritual realm but was a human inspired by God as the greatest Prophet and saviour of mankind.. Then there were those that saw Jesus as actually a part of God. The problem was the early church had to define what Jesus actually was, God incarnate, just a human, or something else. If the early church was praying to him, he could not be another God, as that would make them Polytheists, so it was devised, that he had to God, but then how to explain that and still be monotheists? In comes the Trinity/Godhead, so it was decided and all the bickering about what Jesus was ended (officially). All written works from other traditions were burned, and those that did not conform were either burned too, or were exiled.