As an agnostic, I would offer the following as a pretty good reason not to believe that God exists, and for being an atheist (were I so inclined):
If God exists, and if one of God’s chief purposes for creating the universe was to bring forth life on Earth and beings with minds (that is, us), then that life and those minds have been, to put it politely, rather at their leisure in arriving on the scene. Scientists put the beginning of the known universe at 13.7 billion years ago, and our Earth’s formation at 4.5 billion years ago. The simplest microbial life appears to have started on Earth about 3 billion years ago. In other words, more than 10 billion years passed before life on Earth even got started, and in terms of minds, humans have only been around for about 100,000 years. Isn’t this a rather roundabout way for God to arrive at his chief purposes? If, afterall, a mind with infinite powers is really at work, would that mind be so, well, serendipitous? It’s as if you had access to a hammer to build your house, and knew quite well how to use a hammer, but on a lark you chose to use the tip of your nose to tap in all of the house’s nails (which I presume would make the work proceed very, very slowly, and with a great deal of unnecessary suffering). Given your abilities and tools, it would seem to be a rather messy and superfluous way to achieve your larger purposes and designs. It’s certainly possible that God is an aesthete, devoted to creative whimsy and excess (think the peacock here, and Michael Jackson), but it cannot help but make you wonder and doubt what God is thinking. If an all powerful mind really is actively engaged in directing us into existence at this late and, by all appearances, contingent point and time, one is left puzzling out to what purpose.
It would seem, at minimum, that the relatively late date for the appearance of our Earth, life, and mind in the universe is a perplexity for normal theistic expectations, and certainly poses no perplexity at all for the atheist. Indeed, it’s exactly what an atheist would expect to find in a universe containing life and mind, but is absent intelligent design.