Because it doesn't make any sense that there should be an omnipotent, omniscient, immortal being in the world. Everything in the world dies, and more to the point, the world can only function because everything dies. [breaks into 'Circle of Life'].
Because the idea of an afterlife is also awful, and risible. I don't want to go on for ever. And again, the idea of humans lasting forever doesn't work, we develop, we age, we die, that's how we function. If something in us lasted forever, we wouldn't be human.
I certainly can't believe in an interventionist God, frankly I find the idea a little sick. I have attended church services where the answer to 'why did X bad event happen' was 'because the person/family/tribe didn't pray hard enough'. Seriously! So apparently their God will decide whether or not to save/protect person Y because they had 30 people praying for him, but will not save person Z because they only had 10 people praying for him. How can they actually believe that??
I have been open in the past to the idea of a God who is not powerful or able to intervene, but who is with us in our suffering. But really, that does seem like wish fulfilment too. Currently an atheist, as perhaps you can tell.