"Why Do Buddhists Not Believe in God?"
Buddhists don't believe in God? I think that's a hard statement to make, even though there are plenty of Buddhists and non-Buddhists who are perfectly comfortable making that statement. For me, it depends on what you call "God".
The question might be loaded with assumptions and preconceptions about God, and quite honestly, I don't think there is even full consensus among the major Western traditions through all of history about the nature of God.
Buddhist might easily believe in God. There are ideas and concepts in Buddhist thought that when described in particular words, match some of the descriptions of God by writers in the Western traditions! It's just that Buddhists might not call it God.
A short while ago I wrote about a discussion I had with a youngster from Berkeley who was over here converting the heathen and made the mistake of approaching me. He got an earful from me. At one point towards the end of our discussion, I think I asked him, "Do you think I believe in God?", and he thought for a few seconds and said, "Yes, I think you do".
I took that as high compliment, as well as a reason to respect him. He had just gotten a barrage of my alternative theological theories, none of which should have led him to think I believed in the Christian formulation of God, and here he was saying that we have this belief, for him a sacred belief, in common.
I think it was enough for him that I had given these ideas so much thought, and though I told him I was coming from a scientific background (I didn't mention Buddhism at all), there was obviously consideration of what he calls "divine" in my formulations.
I am perfectly comfortable with someone taking my formulations and interpreting them in a way that says I believe in God. Because in those interpretations, I do! It might not be my first choice of language, but if they're the ones saying it, then it's their language they're saying it in, and I believe in God! I have no problem with that.
As for Buddhists "who don't believe in God", and I'm trying to be careful as I would encourage them to be careful, I would caution about further believing that people who believe in God are wrong. I think that should be a hard statement for a Buddhist to make. What's wrong, what's right?
Are Buddhist views wrong or right? No, they're just views. If someone thinks you're wrong, fine, maybe they're right. Who knows who's wrong or right? You can only go by what you feel and what you believe. That's something Buddhists should cultivate and know because it is the more compassionate and peaceful approach. It is not compassionate or peaceful to think someone else is wrong.