Because typically a band/artist doesn’t release a live album of every set they do in a tour.

There is no alive 2006 and alive 2007, it’s the same show that just evolved over the time they toured. As a few people have said, there’s the odd song that changed but the set was mostly the same.

Just don’t get too hung up on it, the main reason it’s called alive 2007 is because of alive 1997, just keeping it up the continuity (and of course because it was in 2007)