Well, the main thing is lack of sufficient reason to think any of them are real. But some of us are curious to know by what criteria theists believe one or only some of them are real and the rest are not, but find some people not believing in any of them unfathomable.
We know why we (atheist rationalists) don't think they're real: lack of good evidence, lack of falsifiability, and lack of arguments in favor of at least one that are both sound and valid.
We don't know why you don't think the ones you agree with us aren't real, aren't real. Well, I personally think it's because that's what your religion teaches you, but a lot of theists don't seem to want to come right out and say that (understandably so, I think).
We know why we (atheist rationalists) don't think they're real: lack of good evidence, lack of falsifiability, and lack of arguments in favor of at least one that are both sound and valid.
We don't know why you don't think the ones you agree with us aren't real, aren't real. Well, I personally think it's because that's what your religion teaches you, but a lot of theists don't seem to want to come right out and say that (understandably so, I think).