Because i never faced it! SIMPLE!!
You guys are claiming it just after seeing a broken axle without knowing the exact conditions and without talking to the driver of that particular car.
And iam claiming on the basis that i keep a 8G civic since 3 years and i never ever felt any slight difference in its original performance when it comes to axle.
Lastly, there is a thread named "Problems in 1.8 Civic". There we have discussed every problem about civic from its sluggish 2nd gear to its engine mountings. So your axle claim is not possibly acceptable as you have never faced it nor you are sure about it.