Look into Kurt Gödel's incompleteness theorem, and the philosophical implications of that.

A lot of times, when we're dealing with the assertion that we don't have free will, we're analyzing that according to rule-based systems. The system that we use to evaluate truth isn't entirely rule-based, and is necessarily a superset of what we can consciously evaluate.

In effect, some less-complex system that is a subset of your larger mind is saying 'you have limits, and they are this.' But your larger mind disagrees, because that more rule-based subset of rights is incapable of knowing the limits of its superset. Though, we just feel like it's 'off'.

If it feels like it's off, there's a good chance that the overall way you're thinking of it isn't right, even if the literal thing you're focused on has some degree of truth.

In short, it's possible to know something that is technically true, but that isn't interpreted correctly internally.

If you accept the model that you have no free will without processing the larger feelings it evokes, then whether or not your internal sense of free will is rule-based, you'll artificially limit the way you think to filter out the internal process you think of as free will. ..and that can have massive consequences for your happiness and viability as an organism, because you've swapped away that which you actually are for labels and concepts of what you are - but your concept is fundamentally less complex and led capable than you are as a whole.

Fortunately, rule-based systems break when faced with reality. It's just that it can be very painful to go through that process with what you identify with.

Things can be true on different levels and false on others. The earth is locally flat, it is as a whole a near sphere.

I don't know if we have free will or not, I strongly suspect that physics can explain our minds fully, but I don't know. At the same time even if physics could fully explain our minds in practice we are so complicated we give the impression that we have a limited amount of free will. So yeah the earth is round but it is easier for us to assume flat most of the time.

We don’t have a free will.

We do have a free won’t.

That's a very large assumption. The simplest explanation is that we feel like we have free will because we do. Quantum mechanics suggests some major challenges to determinism, and the best arguments to restore it require a very unsatisfying amount of magical thinking.

Because maintaining the illusion keeps us going as normal and won't break the simulation. /s

I don't believe there is an "illusions that we have free will," either. Honestly, "illusions" don't really even exist as they're traditionally talked about. People say if you place a stick in a cup of water, there is an "illusion" created that the stick is bent. But is there? What you see is just what a non-bent stick looks like in a cup of water. Its appearance is different from one out of water due to light refraction. It's not as if reality is tricking you by showing you a bent stick when there isn't one, that's just what a non-bent stick in water really looks like.

The only "illusion" is your own faulty interpretation of what you are seeing, which upon further inspection you may later find it is wrong and change your mind. There was simply no illusion there to begin with. Reality just presents itself as it actually exists, and it is us who interpret it, and sometimes we make mistakes and interpret it wrong. But it's not reality's fault we interpret it wrong sometimes. Reality is not wrong, nor is it right. It just is what it is.

In a similar sense, there is just no "illusion of free will." Neural networks are pattern recognition machines. We form models of the external world which can approximate different counterfactual realities, and we consider those realities to decide which one will optimize whatever goal we're trying to achieve. The fact we can consider counterfactual worlds doesn't mean that those counterfactual worlds really exist, and indeed our very consideration of them is part of the process of determining which decision we make.

Reality never tricks us into the counterfactual worlds really do in some way exist and we are selecting from these possible worlds. That's just an interpretation we sometimes impose artificially, but honestly I think it's exaggerated how much of an "illusion" this really is. A lot of regular people if you talk to them will probably admit quite easily that those counterfactual worlds don't exist anywhere but in their imagination, and that of course the only thing real is the decision that they made and the world they exist within where they made that decision.

Hence, reality is not in any way tricking us into thinking our decisions somehow have more power than they really do. It is some of us (not all of us, I'm not even convinced it's most of us) who impose greater powers to decision making than it actually has. There just is no "illusion of free will," at best there is your personal misinterpretation of what decision making actually entails.

I can’t think of any good reason why we would have such an illusion.

Why are we assuming we don't have free will? We do. Its not total freedom, our freedom is contingent on existing circumstances, but hard determinism is easily disprovable.

The idea that there is no free will is a mind fuck that keeps you from questioning your reality. You might as well ask, "assuming the earth is flat, why does the stick disappear on the horizon?"