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Why do most undergraduate textbooks not contain solutions?

I'm a first year undergraduate. My preferred method of study is through textbooks, however I've noticed that a lot of undergraduate books do not offer solutions. I understand that the greatest value is in solving a problem yourself, and I never look at the solutions before I have solved a problem, but it is very helpful to see the 'expert' solution. Additionally, without solutions how can someone know that they haven't made a logical slip somewhere in their proof?

Can authors just not be bothered or is their a valid reason for not including solutions?