Earlier quoted context omitted.
They didn't. Many insects are even literally kosher. It's just that farming them didn't make much sense until recently. Plants and mushrooms, having far more different biology from us than insects, are probably a worse concern when it comes to toxins that can't be cooked away. And yet humans managed to cultivate food with plenty of those, like cassava.
If by many you mean just locusts. When a bunch of bugs eat all you food and the only thing left to eat is those bugs, then I think locusts being kosher has more to do with survival than actually being kosher.
Either way, Jews and other middle eastern groups with picky dietary rules, do have a long tradition for eating it in non-desperate times too.
It's icky for us not used to it, but there's no reason to think parasites is a bigger issue with insects than with animals far more similar to us, which we do eat.
Remember this article is about pet food insects, often fed to pets raw (if not alive), and it notes some extremely unsanitary practices in pet shops that farm them, such as feeding them on the excrement of the type of creature they're fed to. It's not particularly relevant for insects considered for human consumption.