Because, as I said, it's a manufactured book, not one put together by the singer.   And since it has lots of songs, it's easy for somebody to decide on the spur of the moment they'd like to try one--having no idea how it goes.   And everybody else is subjected to that.   Or they stumble through every verse.   And if you sing a song that's in that book, but not the exact words as in the book, or in the same order,   everybody with the book is baffled--and not happy.

There is also no reason on God's green earth that everybody has to sing every word of every song. I'm perfectly happy to just come in on a chorus, and let the leader sing the verses.

Again, if RUS floats your boat, no problem. Just tell us in advance that that book will be there, and those of us who want a different experience will make plans accordingly.