It is relatively easy to determine whether through using a particular definition God does or does not exist. For example, if we imagine God to be a senile old guy with a white beard who farts thunderbolts, then most people can clearly and with confidence say that such a God doesn't exist (except in our imaginations).
Throughout the ages many have made the argument that combining high and noble concepts of the human world with such insanely useless visualizations of God is best to be avoided. (Although, doing it as a short term mental exercise can be beneficial.) The worst thing is that many people are willing to give up self-development and thinking on their own because of such questionable depictions.
The modern day mascot of this freakshow side of God is the Flying Spaghetti Monster. The argument is that if you can imagine a vengeful, delusional old guy for God, I can imagine a flying spaghetti monster and neither of us will be more knowledgeable of the world or a better human person in general.
Why doesn't God exist? Such a God doesn't exist because we humans as a collective are unable to imagine a God that would live up to our expectations. In other words, confused minds generate confused ideas, and unless we come up with a useful, commonly accepted use of the word 'God', that confusion won't go anywhere and confused ideas will just keep multiplying.
What can you as an individual do? Strive for clarity and don't get stuck in the limitations of words.