For the same reason you'd default to the position of not believing my claim that, say, I have an invisible pet dragon, or that I have a scientific theory that is better than Einstein, or that I'd been visited by the ghost of Elvis last Saturday afternoon.
You couldn't actually know that any of those claims are false, could you? So you'd have to, if pressed, be "agnostic", but I very much doubt that you'd believe me, so you'd be an a-dragonist, and so on. That's how you get agnostic atheists. Agnosticism is about knowledge and atheism is about belief.
This is not about you per se. It is about some, but not all of the non-religious humans
That is a part of it. But not all - that it is the unknown, that is the tricky part.
You only know first person what you experience in your mind. You then properly claim, it makes sense to add "objective reality". You then add that you can trust your sense and reasoning and decide what, that which is independent of your mind, is as independent of your mind. But you see, that is unknown unless you are outside your mind and that is not the case, right?
So you have some basic beliefs, that you trust in, that it is so. That is what in effect you do, yet that trust, namely that you can trust objective reality to be what you believe it to, is not like religion at all. You don't really trust something bigger than you, objectively reality, so be fair(not a Boltzmann Brain, yet natural) and be independently of you as it appears to you in your mind.
No, you use the correct reasoning, because all other reasoning is incorrect, because your reasoning is not based on faith at all. That you can trust objective reality to be fair and natural and that you can trust your reasoning and senses is not anything like religion and it is not like, that what I call God, is what you call objective reality? No, not at all!!! Because you are special, you know that objective reality is real as you claim and that is not a belief.
I don't know if this applies to you. But it applies to at least one of the members of this forum.