Much of theoretical science is not based on anything but speculation and educated guessing. As for "reason"...that is wide open to interpretation as well. My reasoning is not your reasoning. We shall all find out sooner or later whose position was reasonable and whose wasn't.They are not founded on science or reason. They sound reasonable, on the surface, but they are scientifically unsupported.
You seem unable to reconcile yourself to the possibility that there is no purpose, or that "fulfillment" is a product of your own efforts.
Nothing happens for no reason. Cause and effect states that whatever you see happening is the effect of a cause.
The Bible gives me purpose and reason.....it is very acceptable to me, but it might not be to you. That's OK.
Why do you feel there must be a 'why?'
Because that is the way our brains are wired. What do you think science is for? Isn't it to formulate a reason for all the "whys"? You turn to science for your answers....I turn to the Bible for mine. You have questions you can't answer because science can't provide anything but suggestions on many of its theories....but I don't have gaps in my understanding.
It is actually very satisfying not to have to rely on man for the future of planet Earth.