I think one reason is, kids are forced to read things by other people. One of the quickest ways to get me to lose interest in something or actively dislike it, is to tell me I have to do that thing or else. Kids need more independence and time to discover things on their own.

When you're assigned reading and you hate it, that's what you remember. It will just be one of those things you had to do, that once you're in adulthood you're grateful you don't need to anymore. Kids need to be empowered to make their own choices about what to read.