So a couple of things on this:
a) The giant theropods and sauropods don't have hips that resemble bipedal humans. This allows their load-bearing machinery to function in proportion to their insane loads while also interfacing with their spinal column/necks/tails to distribute those insane load such that they can move just fine...however, they could NOT walk/load-bear in an upright fashion. The premise of "dinosaurs/T-Rex could do it, therefore massive upright bipeds should be able to do it" is not a line of evidence. Its actually the exact opposite. If Giants' endoskeletons possessed similar architecture to distribute their massive load, they would look and behave nothing like D&D giants. They would look like alligators...and have massive tails...and long necks or massive heads.
The beauty with Evolution is that it really does not care what either Manbearcat or Shasarak has to say on the matter. You could certainly argue for Alligator Giants if you prefer and on the other hand simple biology is able to meet every structural requirement that you mention. It turns out that bone can handle a giant weight without breaking, respiratory systems can handle the air requirements of giant breaths, ankles can be reinforced to bend correctly, temperature to be regulated correctly so that really the only thing missing is another creature big enough to meet the caloric requirements that a Giant would need.
b) There is no indication that the overwhelming number of medium+ sized creatures with exoskeletons in D&D (the arthropods) have magical respiration or kinesiology or load-distribution. We apply so many earth-based physics and biological (specifically how their form and systems relate to gravity and atmosphere) bounds on martial heroes yet the exact same limitations that should disallow spiders, scorpions, ettercaps, umber-hulks (et al) from being larger than a chicken are hand-waved away...because "reasons?"
Personally I would like to see the Umberhulk that is the size of a chicken but in truth it would most likely turn into another Gopher type creature forever digging holes in the lawn.
c) Evolution is not a thing in D&D land. All creatures are basically magically spawned via primordial forces or brought into existence via divine myth (eg a God bled or cried them into existence, etc). So why are we inconsistently applying selection pressure-based evolution to adventurers/martial heroes or applying earth-based physics/biology, yet ignoring one or both of these things for the many fantastical creatures they face in battle (and must move dynamically to do so!)?
I think that if you look at the Kobold for example you would see that Evolution is alive and well within the DnD Universe.