Because it's highly unlikely that any but the very newest players will only have this book to go on. There is a ton of online support through reddit and other forums, blogs, and videos or podcasts, or even just personally knowing more experienced gamers. And if someone is so new and inexperienced as to not realize they can change monsters as they like, then it's possibly better for them to not have a more complex monster to confuse them. Once they gain experience, they'll be able to change what they like on their own.
Don't forget that throughout (A)D&D's long history, monsters were rarely complex in a good way. They either had very little to go on or were given confusing or boring abilities. And yet, many DMs have been able to play them well. So don't write them off now.