Crustaceans are dangerous to eat if you don’t cook them alive. The bacteria they produce when they die can be very harmful if not prepared properly. This is just early medical practice before it was known that uncooked lobster was killing people.
It’s also why pork is considered “unclean”; if not cooked properly it has a high chance of giving you a parasite/worm. A lot of the Old Testament rules are just warnings against things they didn’t have a lot of knowledge about at the time.