Why do absolutes have to come from somewhere? Why can't they be natural? (that's what the writers of the Constitution thought.)
That is incorrect, the writers of the Constitution were mainly Christians with a smattering of deists and all penned their names to the Declaration of Independence acknowledging that rights were unalienable and granted by the "creator".
Well as I see it there are a few choices. Either they simply materialized out of thin air. Sort of like an ever expanding universe that came from nothing absent a catalyst.
Or it could be that moral absolutes simply assembled themselves like the monkey typing Shakespear on a computer given enough time.
Or they were God given. The only one that makes sense to me is the final one.