I think you answered your own question in the subject like of this thread.
Why does God exist? Because people's minds make him real to them.
Why is he in everybody's mind? Because religious people can't keep their goddamn mouths shut and keep talking about God.
God's is in our mind because in our society from the small age only a innocent child is taught that there is god and he make this universe and he is within them, always protect them , helps them from any kind of bad situation,danger. After that child grown up and become conscious he may think that god doesn't exists,why all people are behind god, who is god but he can never eradicate that faith toward God from his mind taught by society which let him to believe that god may exist.Why does God exist? Because people's minds make him real to them.
Why is he in everybody's mind? Because religious people can't keep their goddamn mouths shut and keep talking about God.
And i'm one of them.