I believe @Trispeed@Trisped has the best answer with respect to an in-universe explanation.
However, one overlooked additional possibility is the maximum g-forces that human/alien physiology can endure. Direction matters when it comes to experiencing high-g loads. Unfortunately I couldn't find anything about the effects of transverse (left-right) acceleration on the human body. Most studies seem to be on vertical (aligned with spinal axis) or horizontal (front-back).
Even so, the reason could be as simple as seat design. By banking, the spacecraft will exert the acceleration on the pilots in the vertical direction rather than the transverse direction. Ever been in a packed car that takes a sharp turn at a high speed? Everyone in the back seat ends up squished against the person on the outside. Now imagine if the bank was proportional to the ratio of acceleration/gravity, everyone would be pushed nicely down into their seat instead of leaning on the person next to them. If spacecraft are designed to bank into turns, the seats don't have to be designed to allow the pilots and passengers to withstand high-g loads from ALL directions - just two. Normal seat designs already provide stability in two directions: vertical (seat bottom) and horizontal (seat back). By going with this design path, the need to develop a complex (and possibly movement restrictive) restraint system to allow the pilots to withstand transverse acceleration is eliminated.
Then again, if we're assuming that the ship's artificial gravity can magically compensate for all these inertial loads on the pilots, then none of this matters. However, there's no way for AG to compensate for this without defying the laws of physics (at least according to our primitive human understanding of physics). AG acting in a steady state may be plausible - I can buy that in a universe that has tractor beams. However, for the passengers to stay in the same relative position within the ship, they have to be subject to all the acceleration the ship experiences as well.