Do you have a theory of purpose that is consistent with the laws of physics? Because that would be a major breakthrough. All of the evidence (AFAICT) indicates that nature is utterly indifferent to our existence.
Nature may be indifferent to our existence, but we are not indifferent to nature. How is this?
Science leads you to the conclusion that we are here as the result of a series of chance events. Not because of any "decision" to put us here. If we're all just here by fluke, and when we're gone we're gone entirely and completely, what transcendence could there be?
How does science exclude the possibility of a decision to put us here? It seems to me that science (paired with a good bunch of epistemological assumptions) gives us the laws our universe follows. But as for why such laws are like that, or who possibly decided them and with which purpose, I'd say such questions do not even make sense for science.
How does it lead to that? I feel like that's a false assumption that many people make, religious or not.