First of all, You cannot define or imagine a thing into existence.
If you allow one thing to exist without a cause (God), you contradict your own premise. And if you do, there is no reason why the universe should not be the one thing that exists or originates without cause.
To argue against the "designed part": The principles of self-organization and evolution provide complete explanations for apparent design.
To argue against "the Anthropic Cosmological Principle": The odds against all possible universes are equally astronomical, yet one of them must be the actual universe. Moreover, if there are very many universes, then some of these will contain the possibility of life. Even if valid, the anthropic cosmological principle guarantees only that stars and planets and life will emerge - not intelligent life. In its weak form, the anthropic cosmological principle merely states that if we are here to observe the universe, it follows that the universe must have properties that permit intelligent life to emerge.
To argue against a very large number of people claim to have "personal religious experiences of God":
We cannot assume that everything imagined in mental experiences (which include dreams, hallucinations etc) actually exists. Such experiences cannot be repeated, tested or publicly verified. Mystical and other personal experiences can be explained by other causes.
The usefulness of a belief does not prove its truth. In any case, many societies have thrived without these beliefs, while crime has thrived in theistic societies believing in heaven and hell.
Philosophers who have provided arguments against the existence of God include Immanuel Kant, David Hume, Friedrich Nietzsche and Bertrand Russell. In modern culture, the question of God's existence has been discussed by scientists such as Stephen Hawking, Francis Collins, Lawrence M. Krauss, Richard Dawkins, Carl Sagan, Neil deGrasse Tyson, John Lennox and Sam Harris, as well as philosophers including Richard Swinburne, Alvin Plantinga, William Lane Craig, Rebecca Goldstein, A. C. Grayling, Daniel Dennett, Edward Feser and David Bentley Hart.
Scientists follow the scientific method, within which theories must be verifiable by physical experiment. The majority of prominent conceptions of God explicitly or effectively posit a being which is not testable either by proof or disproof. On these bases, the question regarding the existence of God, one for which evidence cannot be tested, may lie outside the purview of modern science by definition.
I say “there is no God” with the same confidence I say “there are no ghosts” or “there is no magic.” The main issue is supernaturalism — I deny that there are beings or phenomena outside the scope of natural law.
That’s not to say that I think everything is within the scope of human knowledge. Surely there are things not dreamt of in our philosophy, not to mention in our science – but that fact is not a reason to believe in supernatural beings. I think many arguments for the existence of a God depend on the insufficiencies of human cognition. I readily grant that we have cognitive limitations. But when we bump up against them, when we find we cannot explain something — like why the fundamental physical parameters happen to have the values that they have — the right conclusion to draw is that we just can’t explain the thing. That’s the proper place for agnosticism and humility.
But getting back to your question: I’m puzzled why you are puzzled how rational people could disagree about the existence of God. Why not ask about disagreements among theists? Jews and Muslims disagree with Christians about the divinity of Jesus; Protestants disagree with Catholics about the virginity of Mary; Protestants disagree with Protestants about predestination, infant baptism and the inerrancy of the Bible. Hindus think there are many gods while Unitarians think there is at most one. Don’t all these disagreements demand explanation too? Must a Christian Scientist say that Episcopalians are just not thinking clearly? Are you going to ask a Catholic if she thinks there are no good reasons for believing in the angel Moroni?
How about you for example explain these SIMPLE things
Because evil exists, God cannot be all-powerful. all-knowing and loving and good at the same time.
Because God allows pain, disease and natural disasters to exist, he cannot be all-powerful and also loving and good in the human sense of these words.
Destinies are not allocated on the basis of merit or equality. They are allocated either arbitrarily, or on the principle of "to him who has, shall be given, and from him who has not shall be taken even that which he has." It follows that God cannot be all-powerful and all-knowing and also just in the human sense of the word.
Since the Gods of various religions differ widely in their characteristics, only one of these religions, or none, can be right about God.
Since God is invisible, and the universe is no different than if he did not exist, it is simpler to assume he does not exist.