Why are there not solutions to all practice exams?  
You don't always try hard to do the problems before looking on the solutions.

If there are no answers how do I know if I got it right?
If you understand it well you will know if you got it right.

Why are there not more practice exams?  
You can learn to solve the types of problems without gaining any further insight. 

What are the practice exams and solutions good for?
It helps you test your knowledge and give you an idea what types of problems to study.

"It is not the answers that show us the way, but the questions."
You gain understanding not from the answers but from the process of trying to solve.