Remember that God created man expressly for fellowship with Him, and he built inside each man a desire for fellowship with God.
Since man is no longer perfect, he tries to fill that desire himself. He tries things he thinks will make him more "spiritual". Many people attend a church because they think it will make them closer to God. Many try to do good works because they think God will be more pleased with them. Many try other religions, and "create" a "god" that they can fellowship with instead of the true God who created them.