Father Mike Schmitz from Ascension Press answers this question quite well. Here is an excerpt:
"To claim to be spiritual but not religious is like saying, 'I'm a scholar, but I don't read.' What informs your scholarship? There needs to be content. Where would a spiritual person get data about the spiritual life? How would they even know the answers to the most basic questions: Is your spirit good? Is God good? Who is God? Is God on your side? Who are you? What is the goal of living?
"There is the desire of 'I want to do what I want to do.' If I have religion, then I am obligated to obey someone else's will. But if I am merely spiritual, there are no demands upon me other than to follow my own sense of 'this is what I currently prefer.'
"I've seen this mindset manifest itself with comments like, 'Well, I find God the most in nature, so I would rather go into the woods on Sunday morning than to a church.' I also find God revealing himself in nature. But what is going on here? I am merely asserting my own preference: I want to go to the woods, so I will go to the woods. I don't actually have to be concerned with what God might want. I'm not even asking if God has a preference regarding where he wants to be found or how he wants to be worshiped. I am merely doing what I want to do.
"In this case, my 'god' is me. I become the one who decides 'this is what God would want,' rather than asking, 'God, what do you want?'
"'Spiritual but not religious' is just another form of idolatry. In this case, the false god is the god I have invented. God is no longer a someone. 'God' is now just some vague idea, an impersonal force that makes no demands on me and is simply 'there' so that I don't feel lonely.
I will talk to people who say, 'All I need to know is that God is love.' I believe that God is love as well. But where did they get that truth about God? They didn't get it from looking at nature. If anything, nature is impartial. It can be beautiful, but it can also be ugly. It can bring life, but it just as easily brings death. There is nothing in nature alone that indicates that God is love. Nature doesn't seem to even care about human beings.
"We have been told that God is love through the revelation of Jesus Christ. No person on this planet had even hoped that God is love until Christianity. There was no proof of this until God became one of us and revealed himself in this way. The only way Jesus could accurately reveal this truth is if he himself is truly God. And if Jesus is God, then that means that God is not impersonal, he is not invented by us; he is revealed to us. Our spirituality has been formed and informed by our religion."