Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I’d like to address: a lot of people are asking ‘Why assume this?’ The answer is: it’s purely rhetorical! That said, I’m happy with a well thought-out ‘I dispute the premiss’ answer.
You experience the world through your senses.
What sense that your body has would you expect to give your brain a different set of inputs if your brain’s actions were not deterministic, not set by the laws of physics? How would you expect it to feel different?
You wouldn’t expect to feel like some invisible force is in control of your limbs, which I think is perhaps what some people intuitively expect if someone says that their actions are pre-determined.
It’s not talking about anything that your brain can sense; it’s talking about how your brain works.
Yeah, this is it.
And to take a slightly different tack, if the biochemical and electrical activity in your brain were not deterministic, how would you ever know? It’s one thing to believe that you made a decision on your own “Free Will”, but how could you possibly rewind the entire universe (or at least some sufficiently small portion of it), including your brain’s exact atomic state, and re-run the experiment to know for sure? At that point, what would “Free Will” even mean?
There are many thing my body does which I’m aware of, but that I don’t will, and others that I have some control over, i.e., my will appears to play a role, but not the only role.
I don’t think it creates any kind of contradiction to suggest that, hypothetically, there could be more (or less) of either of those types of things, without my perceiving an invisible (external) force of some kind to be involved. After all, I don’t ascribe my heartbeat to an external force, but I am aware that I don’t will it.
No, but you have the ability to sense your heartbeat, so you can tell that it’s there.
You don’t have the ability to sense electromagnetic emissions in the X-ray frequency range, so you can’t tell that they’re there. You wouldn’t know if X-rays of a given intensity were present at a given moment. It’s like asking “why is there the illusion that there are no X-rays” when you wouldn’t expect to feel differently regardless of their presence or non-presence.