Very old laws on the books that no one has bothered to change in some states. Federally, they aren’t illegal. Federally, knives weren’t protected under the 2nd amendment so states had an easier time putting rules in place against them, and many did. My state just legalized otf and switch blades for everyone a few years ago. Mainly because unassisted knives were able to become just as quick and easy to open, it was a bit silly to leave assisted open knives not legal.
As an added note just for you: cardboard is hell on knife blades and they’ll dull them very quickly. I’d get yourself a box knife and blades f9r it if you open a lot of boxes, unless you want to sharpen a knife like twice a week.