It almost sounds to me like you are asking "why did the molecules have to reproduce" as if the molecules were intelligent beings that decided to reproduce because they "knew" it would lead to this beneficial thing called evolution. I know you don't actually believe that, but it seems like you can't imagine how it could have happened otherwise.
If, as is almost certainly the case, at the beginning of life on this planet there were molecules that self replicated - reproduced - then it was only because the chemical/physical composition of those molecules demanded that they do so. It would have been a violation of physical laws had they not done so. No intelligence or purpose or grand plan was required.
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