Humans have an absurd amount of endurance. It's really quite crazy. There are few animals that can match a human for long distance running or ability to cover rough terrain. It's a bit of an urban myth that we are the sissys of the animal kingdom who only survive due to our brains.

There are actually some cultures out there that hunt by literally running animals down. They chase them until they collapse of exhaustion.

We have endurance, but can't outsprint any of the great cats or bears.

The Masai don't have to worry about outsprinting the lion, they just carry a BIG spear. ;)

But, yeah, we're definitely excellent persistence hunters.

I don't think you can legitimately ask "Why are humans bipedal?" because the why implies a cause of evolution, the more correct question would be "Why was being bipedal an advantage for the Homo genus?"

While this is entirely speculative, I suspect that early ancestors probably evolved very primitive hand-like paws for grasping and climbing; which was useful for defence (far fewer predators in trees than on the ground) and also opened up more opportunites for food. After this I would expect that we became an aquatic ape that (potentially) spent a lot of time in the water catching and eating fish, potentially while still living on land/in trees; and this would require our hands to work for walking, grasping and swimming. At this point, I would expect that our ancestors were potentially getting good at using sticks and rocks as tools, and this balanced out any loss of speed or strength of having more delicate arms/hands; and evolution began to favour individuals who were better tool users.

Over time, the more successful offspring were the ones who used tools more often; and being more upright made it easier to use tools.

Furthering your point, can that predator climb a tree? Because I can.

Can that predator swim? Because I'm a pretty good swimmer.

Can that predator call in a dozen other animals to back it up? I can.

Humans are adapted for adaptability. We aren't meant to just use armour or speed or strength to survive, we have a variety of techniques that we're pretty good at and we pick the one best suited to help us defeat a threat.

As wise ol' Chuck D. said, it is not the strongest of the species which survives, nor the smartest. It is the one that is the most adaptable to change.

You find out something old actually. Just because you just found out now doesn't make it new.

Absolutely, I was about to say that.

Versatility is very important. We can run, climb, swim, use tools, communicate via language etc etc.

The reasons why humanity has been so dominant is not just because of our brains. Though the logic inherent within us and our ability to transfer information quickly via the written and spoken word has lead to our extreme technological advantage that we now have of course.

Furthering your point, can that predator climb a tree? Because I can.  (a leopard will outclimb you)

Can that predator swim? Because I'm a pretty good swimmer.  (a croc will outswim you)

Can that predator call in a dozen other animals to back it up? I can.  (a hyena will call its pack with a hearty laugh)

Being bipedal provides no advantages when dealing with predators.   

However there is no animal that can do all of the above

Rugby...

Because it's the only way we could walk and pick our nose at the same time.  This multi-tasking helped us beat out quadrapeds while running after food as they would have to stop to pick their nose.