Why believe in a god?
I guess people do it because they aren't Jehovah's Witnesses and aren't under the influence of the Governing Body that tells them what they can and cannot believe in.
Sure, it might seem silly to you and even against reason. But people don't have to listen to you (or anybody, for that matter).
And especially not those idiot men that act like Puppet Masters via the Watchtower publications and tell people what they should and should not wear, should and should not do, should and should not think, and should and should not believe in.
When you don't live under a harsh, ugly, and controlling system like that, you have what is called freedom.
With freedom I can believe in anything that isn't real, like Santa Claus, the Easter Bunny, even the US Congress--well, uh, maybe scratch the last one. But you get my drift.
It's great that you have found freedom and have come to your own conclusions. But in a world of freedom you will also have to get used to the fact that people don't give a flying fuck about what you believe (or don't believe) as long as you respect their customs and convictions.
If you don't, you're messing with that freedom you're cherishing right now. That's why people hate the fuck out of the Watchtower and the Jehovah's Witnesses. They want to put everyone under their mighty thumb, extinguish freedom of thought, destroy other customs and convictions, and just make everyone wear the same uniform and think the same way.
Do you want everyone to think like you? You can't have it both ways. Freedom doesn't allow for that.
Why believe in a god? Because you are free to do so when you are outside of the control of a system that says you can't do anything but their way, that's why.