But we don't learn chi-sau per se as a skill to fight with.
It is like hitting a heavy bag to develop the punch- structure, penetration, etc.. But the bag doesn't hit back or follow you. It is nothing like fighting, unless you're picking on a kid, yet you will fight with the punching skill heavy bag training develops. It is one tool and one aspect. No one who trains it thinks it is fighting nor fights a person in just the same way they "fight" a heavy bag.
I assume because chi-sau is a two-person drill where both may be working offensive and defensive skills which resembles a fantasy fight to the unlearned, you have grossly misunderstood its purpose. That's what instructors are for.
But certainly, there are plenty of lousy instructors (watch WCBlast) who seem to have the same misunderstanding and replace the position of sparring with chi-sau and treat it like fighting, but they are missing the point of the developmental drill and indeed the training process entirely.