Because in a world without the force general grievous is like a god. He is a robot with super human strength, speed, and four bloody lightsabers. Mushashi is dead meat. You seen the terminator movies, its extremely hard to defeat a machine. and general grievous is much taller and more advanced than even the T-X.
Your post lacks sense.
1) Grievous is not a robot. He's a cyborg and needs his suit to live, therefore making him semi-organic.
2) T-X's are actually much more advance, Grievous just used the amor and suit, he didn't have any programming or anything, he was still in control.
3) T-X's are much harder to kill. Grievous took one hard Force Push and lost two arms. T-X's take hundreds of shots to kill, though EMP kills em' quick.
4) How does he have superhuman speed? I might understand strength, due to robotic enhancements, but speed? He seems pretty slow actually, he allowed Obi to do backflips over him and couldn't dodge one saber, yet he has "four bloody sabers."
5) Machines aren't hard to kill, EMP rids of em' almost instantly.