Why are there no human-sized exoskeletons

Human-sized Insects - How is Physical Difference to Smaller Species?

The main limitation on insect size is gas exchange. An insect has no lungs or gills; Instead, it has various holes (spiracles) on the outside of its body that lead into windpipes that branch all over the animal's body and tissues directly supply with oxygen. Insects don't have blood in the traditional sense because they don't need it, as gases only diffuse directly into their cells instead of having to diffuse into the blood first.

Here's the kicker: In places where the exoskeleton becomes pinched and narrow (i.e. joints on the extremities), the trachea has yet to pass. With smaller insects, this isn't that big of a problem, as their windpipes don't take up much space and can be very slender. able to walk through their tiny joints. However, as an insect grows, the trachea must become disproportionately large to allow gas exchange into the limb. Once the trachea takes up 90% of the space within the joint, the insect cannot physically get any bigger because it still has to fit into things like tendons and ligaments. In order for an insect in our atmosphere (low oxygen density) to become human or larger, it must change its entire respiratory and circulatory system. This would call the question into question, because if she changed her biology on such a fundamental level, she would no longer be an insect.