Brakes are being added to actuators but they're more useful for static holds / locking than dynamic balancing. Standing even in humans is a dynamic balancing activity.
replyIt is, but you don't have to do it at every joint all the time. If you lock your knees your front and back thigh muscles don't have to work on balancing in that axis at all.
replyComplexity, weight, failure modes, wear, maintenance, support burden of legacy parts.
replyAlso the brake components are never in the same plane as the drive components, so now you have additional forces to engineer for.
You need to keep balancing fast enough. I don't think a break would give you this agility.
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