In this answer, I interpret the OP's "assuming we don't have free will" as assuming we believe that we don't have free will. Then the question is "Why do we still have the guess ("illusion") of free will counter our stance".
The idea that we don't possess free will (or freedom of consciousness, to be more precise) and that our actions are pre-determined cannot be convincing at own living process because it is a knowledge (set of view). Any knowledge is what was already assumed as a fact and therefore it is not helpful anymore. Knowledge lacks apodictic truth and to become persuasive it must be "handled" again or invented from scratch now.
At the same time, there is an intuition in everybody (which can become more apparent if an individual has sharpened his her momentary reflection) that whenever the moment we make choices or decisions we make it with the -consciousness of ungroundedness present. I.e. we do apprehand directly that nothing, and especially these here givennesses of the situation, are forcing us to select that which we are selecting. The spontaneous ("free") nature of a decision in a structured situation (we are "thrown in") is what makes every decision absurd and makes us responsible. To repeat, it is possible to discover this intuition directly and most of the time, if not ever.
But then we start to explain our made choice (we could start doing it soon too, less than a second after). And sure, we find reasons or causes for it - we invent determinism to ourselves. And past knowledge, both lay and theoretic ("neurons, genes" as one example) here is taken on readily as new, to help for present. I am not to state these explanations are "wrong" (after all, they're useful in praxis), only that they are worldly reasons, while decision making actually takes place via splitting oneself off the world (both "external" and "internal" world, if you are used to make this unnecessary distinction).
A particular phenomenologic-existential viewpoint I am trying awkwardly to express here makes no room for the option that there can be causality bypassing consciousness (i.e. something creeping subliminal/unobserved to create conscious experience). Causes or reasons can exist only for the consciousness. They are like any other phenomena. For example, there exist no other pain besides or in addition to the pain experienced, the pain ontologically is confined to the conscious phenomenon of this particular pain. There is no reality other than human-reality, albeit there can be humanless theories of the world.
We do have "free will" (freedom), it is not an illusion. Assuming we don't have it crashes in the very process of conscious life. But to agree with that, it is important to believe more immediate intuition than knowledge.