Well, they are.
But they shouldn't ninja jump, armor should be too heavy.
Unless they're naked. Which would be scary.
This is 500 years in the future. The armor could be more lightweight than it looks. Plus, I think one of the books said that the armor has built in motors, so that would help with reflexes and whatnot.
But they should have stuck to gunfights, not this hand-to-hand crap. Heck, even if the MC was shooting a pistol bullet to stop a sniper bullet, it would be far better than this. There hasn't been too many animes with gunfights and this could have been very promising. I might even make my own lame cartoon version and it will probably have better fight scenes than this. But I really like the characters of this anime.
EDIT:
The Elites in the books are goddamn monsters. Chief wrestles with one in hand to hand after the immediate plot of Halo and gets the stuff punched out of him, yet the game made them rather weak.
The books have to make everything 10x as hard because if they didn't, the plot would suck. Would it be really interseting to read, "Then the Master Chief took out 40 grunts and 2 elites. He ran to the next room and killed 5 elites, one of which was cloaked. He took the sword. A hunter ran into the room. He killed it with the sword. He tea-bagged the body."