Why do I not believe in a God? Because believing never made it so and if the existence of a 'God' is as important as it is asserted to be than knowing and understanding what it is that you believe and how you know it to be true is no less important. To believe arbitrarily in the arbitrary is a potentially dangerous proposition in proportion to the ramifications of what that particular belief entails.
As humans we have evolved into a complex species whose survival strategy rests in and has come to rely on our capacity to know understand and meet the demands imposed on our survival and well-being by reality through a process of reason, if not your own than someone else's on whom you have imposed the burden, not some invisible agent who lives in the sky but another responsible thinking caring human being. We exist as humans not simply because we can think but because we do think . . . objectively and rationally. We exist not by believing what we like but by knowing what we must and thereby believing and doing what we should.
Proof that ‘God’ does not exist as other than the product of an unbridled imagination unfettered by the knowledge which contradicts such beliefs is contained in the ineffability of what ‘God’ is alleged and supposed to be. Consciousness, perception, reason and intentional purposeful creation are the products of living organisms which possess the physical means to carry out the cognitive processes required to achieve consciousness, perception, reason, and creativity, means and processes not only made possible by the existence of such highly evolved organisms but on which their continued existence and further evolution rely.
The ‘reasons’ typically given for believing in a God, of whatever the chosen flavour, are proof of nothing other than that the believer has not yet fully grasped the nature, means, and processes by which we became that which define us as intelligent rational beings worthy of the distinction of that which distinguishes us from all other animals . . . reason. A ‘reason’ is not an excuse to believe what you choose but the process of non-contradictory identification and integration of acquired knowledge by which we determine and choose what we should believe, a process which at every turn in our accumulation and integration of newly acquired knowledge ultimately leads to the only rational conclusion that can be made regarding God, that He can not and therefore does not exist.
I don’t believe because reason informs my beliefs and with regards to God tells me I should never attempt to substitute faith for doubt in the pursuit of the very knowledge which informs me what I should never believe . . . and why.