Why do you think it’s illegal for people to pirate or plagiarize other people’s work?
Pirating and Plagiarizing are basically in the same family. They are stealing someone´s work and making it yours. Or taking other´s work and publishing it illegally or without permission. These things are wrong and illegal because it´s basically like stealing. Or robbing. For example, let´s say you robbed a jewelry store. That´s bad because you´re taking something that is not your´s without permission and without paying. Which is why call it pirating because pirates used to steal gold and goods. If you want to take or use something that isn´t yours you should site it.