It's SOOOOO much easier to make a quadruped robot than a functional biped, especially one with the posture and balance of a human.
The pack mule was just for reference
~but I don't see your point? Fallout is set in 2077, and FO:NV is set two centuries later.
Some powered suit do that, but you haven't explained why a human can't add his own strength to the suit. Like I said before, since the motion of the user isn't restricted (or not restricted much) there's no reason why he can't apply his own strength as well as the suit's strength to lifting/hitting/whatever.
No need really. Consider what it does and why... It carries, and the operator does not feel the weight. This means a 90Lbs. person that can't do a pull-up, can walk cross country with 200lbs of gear. In this case adding is pointless; but in other cases... Consider if it were based on a gearbox... The person inside can't force a gearbox without damage, and (even if possible) setting the gear to some kind of 'Neutral' would remove the armor's strength completely; if it were hydraulics and not gears... well you can't force those without damage either.
**Also consider that this machine is computer controlled and likely won't lift beyond a safe weight.
***Also (again)... if the design incrementally pushed the joint forward, but allowed the wearer to push it farther if they could... it would not be very safe at all, and might not have any force (or control) over the joint suddenly moving in the opposite direction.
Why? If I took a car with a stick shift out into a parking lot (I don't know how to drive a manual), odds are after two days of practice with no training I'd figure it out and know how to roughly drive a stick. Of course, I'd have done some serious damage to the engine and wheels, I'm betting, but I doubt specialized training would be required. I don't see how this can be too different and still have people in power armor able to move with any kind of fluidity necessary for battle.
This is a bit different. One can learn Stick-shift, but it might be an expensive lesson requiring a trip to the garage and a mechanics fee.
Operator training for PA, [assumed] would involve the obvious and the non-intuitive details of its use, (perhaps including the default password if any, and how to adjust it to better "suit" the wearer). It could be similar to a kid skilled in the tricycle, trying to learn the ten (or 21) speed.
I posted a video a few back, that showed a complete novice wearing a prototype suit. He could move it, and probably lift a heavy load with it, but only because it was already running and setup for him to use.