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@janesix
Lot's of reasons.
1. The only evidence we have of free will is the innate sensation that we are somehow in control of our actions. But we're not. At least not consciously. The conscious control you think you have over your own body doesn't exist. Studies have shown that conscious awareness of your actions comes after your subconscious brain has already decided on a course of action. To the extent that you can monitor someone's brain activity and accurately predict what they are about to do before they are consciously aware of it. (Example: https://www.ncbi.nlm.nih.gov/pubmed/18408715)
2. "Free will" would have to be able to operate contrary to the laws of physics. Our brains are made of matter and therefore a subject to the laws of physics and there is nothing about the brain that seemingly defies the laws of physics. Since there is no "free will" law of physics, there doesn't seem to be any allowance for us to operate with "free will." While our behaviors may seem complex and unpredictable, there is nothing to suggest they are anything other than the complex interactions among atoms.
3. Relativity demonstrates that the past, present, and future, are relative. There is no objective, singular, instantaneous, universal "present." Different observers can disagree on whether two events have occurred simultaneously and there is no measure to say that one person is objectively right and the other objectively wrong.
Consider the event: "The pitcher threw the ball." For one observer that event may have happened in their past. For another, it is happening right now in their present, for yet another it could be an event that has not happened yet. The traditional view of time is that events in the past are no longer real, and events in the future are yet to be determined, and only events happening in the present are "real." Yet it would be absurd, given our example, to say the exact same event is both real, not real, and not yet determined. The only logical response is to accept that all events are real, regardless of whether or not they occur in someone's past, present, or future. The idea of a singular "present" is then just a fiction of our mind.
The past, present, and future for the entire universe then is "real" and predetermined. Our observation of the universe is just one among an infinite number of potential points of view observing a snap-shot or "slice" of the universe from a given perspective at a given time. Your birth, your present, your death, are all real events that cannot be changed.