Because as you bank the aircraft you need to apply back pressure on the yoke to maintain altitude. Allowing the autopilot to do this without giving it the authority to limit the bank just seems like a bad idea. You can find yourself in an accelerated stall and even enter a spin rather easily if you bank hard enough with the autopilot trying to pitch up to maintain altitude. The aircraft might reach the critical AoA. All it would care about is doing it’s best to maintain the altitude.
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