I believe they do this because the real target audience is other professors. The author wants those professors to use their book for their own courses, and a bu

I believe they do this because the real target audience is other professors. The author wants those professors to use their book for their own courses, and a bunch of problems with no answeres saves a lot of time. There seem to be very few books written with the autodidact in mind. Sometimes you can find the solutions manual via eBay or torrent though.