The short answer is because it would scare the pants off of us. But if you must know the real scientific answer, well, nobody knows actually:
The short answer is, researchers don't know exactly, although there are several hypotheses as to why insects and other arthropods don't get bigger, said insect physiologist Jon Harrison, at Arizona State University in Tempe.
The first hypothesis is that insects' exoskeletons may not be strong enough to allow them to get much bigger — that they'd have to become impossibly thick. Harrison learned this theory as an established fact during his training, but little experimental evidence to support the idea exists, he said. The only study to look at this question found that larger arthropods don't have thicker exoskeletons, he said. "So there's no direct evidence for this," he said.
Douglas Main of LiveScience explains: Link