Several considerations for transport jets ..
(a) best climb performance is somewhere around 2-3 degrees bank (similar for any multi) ... not all that easy to fly in a high workload situation ... most of us oldies have trouble enough seeing the ball, let alone guessing bank angles between the marked index positions ...
(b) the variation in climb performance for small bank angles is not overly great
(c) some gyro systems will re-erect to a false vertical in prolonged flight at small bank angles
(d) the reduction in manipulative workload while handflying, I suggest, is the main driver for flying wings level. If the boxes are flying, then there is no reason why the system can't go for the best attitude.