Because they have identified a process which can be shown mathematically to be able to produce such structures. Intelligent origin is also not outside of the realm of possibilities, but so far there is no evidence of it and no demonstrated need.
I'll admit this is new to me, that someone worked it out such structures are mathematically possible. Although, producing the necessary structures and information aren't necessarily the same thing. Supposed they found a non-intelligent source that produces books. There would be no reason to suppose the books weren't filled with gibberish. If there was a non-intelligent way to produce a strand of DNA 250,000 bases long there would be no reason to assume it would code for any actual proteins.
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