Because books with solutions give a false sense of security. The goal of exercises is to have the students to think for themselves about the solution. It really

Because books with solutions give a false sense of security. The goal of exercises is to have the students to think for themselves about the solution. It really doesn't matter if they can solve the problem perfectly. People concerned about solutions are in fact thinking about a quick way to get the best grades for an exam. Which is fine, but it should not the first goal of anyone writing a text book on a mathematical topic.