Well, here is a good reason I think:
Man is a creature of sense. All information comes through the senses, the eyes, ears, mouth, nose and hands.
But the things that make God what he is have never been experienced by anyone.
God is by definition, Omniscient, Omnipotent, and Omnipresent. Never has anyone experienced these things. THere is no physical example of these things in the universe; All-Power, All-Presence, All-Knowledge.
THerefore, how do we conceive of their existence?
Because We were fanished by the hands of the omnipotent one, we were given the gift of life by the omnipresent one, and we were endowed with a Rational SOul by the omniscient one. We have the Fingerprints of GOd on us, and they mark us to our core.
Consider also: Truth. Truth is omnipresent: It exists everywhere, and Every-when. In all times and all places. Can you travel to a place where truth does not exist? Can you think of a time when truth did not apply? No. Therefore Truth transcends time and space and matter.
But no effect is greater than its cause; therefore, what Caused Truth? THe Truthful one- God.
THese are some reasons I believe. Because it is reasonable.
Consider this: Imagine created forms, beauty. Tell yourself: But these are not God. Move higher up, imagine heavenly bodies, stars moon, yet still; These are not God. Now imagine the furthest depths of space, the vastness; And say to yourself this is not God. Now imagine, insofar as possible, the Void, the blackness of Space, the stuff between the stars; But this is not God. Imagine the darkness of the minds eye, the ignorance that lies within you, the supreme lack of knowledge, and the simple darkness of your closed eyes; And this is Not God. Eventually, your mind will hit the limit, like a black invisible wall, and you will know that God is just on the other side. Maybe he will open a chink in the wall. And you will be blinded.
By Negation, we run into his fundamental reality and infinity of being, He is being itself, pure act, pure being…Hammer at that wall, and he will hear you.
This is what St. Augustine did.