Because to admit that there's free will is to admit that there's something above physical laws.
That's not an assertion, that's a definition.
The truth is that we cannot know if there are things that are 'above' these laws, because we are not capable of observing outside of these laws. You are eyes are made of matter and mass, if something existed that was fundamentally different, how could you deal with it with your eyes? You can't see it or observe it.
Anyway, with respect to the topic: you're suggesting that there is the possibility of a higher level of knowledge beyond what we can observe with our physical eyeballs. This is a fallacy that goes against everything science stands for. Science is the platform upon which you build models that describe the physical world.
For example, if we witness the effects of gravity occurring over and over in many experiments, then we can create a model of what we saw: the Law of Gravity. Then we can make the assumption that whenever there is mass, there is gravity. So far, it works. If something can be physically shown to break/violate the law, then it will be reexamined and the model will be changed to satisfy the new information.
There is no evidence of anything above or beyond the physical/theoretical models of science. To suggest there is implies faith.
Are you religious, maxklein?
Observation is inherently limited by our observational capabilities, and this is not a radical thought, it's a very very basic thought. So your argument is not just wrong, it's incredibly wrong, and you should not be making it.
For example: Electrons. To posit that there are electrons, but viewed strictly from a 16th century perspective is a position that cannot be observed based off the technology available to them at that time.
As a purist, I take offense when anybody tries to claim that observation and experimentation can be anything but assumptions. It is something we observe to be a certain way, but very restricted by our particular viewpoint.
You think you are a man of science, but what you actually are is one of those people who said the earth could not be round because it looks flat. I've said something that is perfectly realisitic if one has some imagination, and you immediately shoot it down because it does not conform to how you think scientists should think.
Free your mind, and learn to deal with uncertainty.
Electrons are small and you can't see them with your eyeballs. But J.J. Thomson used models of how cathode rays work and how electrical conduction works, and he hypothesized and then experimentally showed that electrons exist. They may not exist, and any good scientist would be open to that possibility, given evidence. But the best models we have today show that they do exist.
Observation and experimentation are one level beyond assumption. They're assumption backed by evidence. Blind assumption is to "suppose a case is true without proof."
Imagination drives science, true. But nothing imagined is ever considered science, or even possible, without some kind of evidence.
Uncertainty is fine. Is there dark matter? Is it possible to travel faster than the speed of light? Why are photons both particles and waves? Uncertainty is what makes these the interesting questions in science.
UPDATE: There are cases where a patient experiences loss of brain function leading to an inability to make decisions. They require a guardian to make even the most trivial choices for them (e.g. what to wear, in what order to eat the items in their meal, etc.) What exactly have they lost? An answer to this question, scientific or otherwise, would be relevant to free will.