Movies are NEVER as good as books.
The producers aren't really doing anything wrong. Their job is to interpret the book and convert it into an hour and half movie. The Percy Jackson books would probably equate to 4 hours of movie time. Most people would not sit that long and the expense in making a movie that long would be outlandish.
I noticed myself when reading a good book I have a tendency to really associate myself with a lead character. I will think about that person and think about their personality and honestly study them through the words written by the author. Authors have time to really develop a character when writing. They can explain deeply the about a characters past and how they work with the plot. The places and settings in books are allowed to develop also.
Movies are so different. When they purchase the rights to make a movie about a book they often ask for the author's input but they don't allow them to take over the task. They are hedged in by time and costs. Movies needs to be made in about 8 months to make sure the audience is still interested in the subject. They probably make 20 hours of footage and drop 18 hours on the floor. Money is a big constraint. The cost of an author to make a castle for instance on 2-3 pages cost next to nothing, but a movie set can cost 100's of thousands of dollars. There is CGI costs in most movies that are quite expensive also. A good lead actor can make or break a movie. This is a crazy example but what if they hired Johnny Depp to be the lead in the Percy Jackson film. It would never work. Depp has an acting style that would never match the role. They must make a profit to make the investors happy or no job for you next time. The list goes on and on.
Another aspect of books and movies is that when I read and book and you read a book we might see it very different. We are both happy with the book and we can discuss it with other and still be enjoyable. The job of interpreting the book goes to one person or small group of people. Now their vision has to appeal to the masses. That might mean tossing out some really good points of the book that you and I might really like. Its a sacrifice that has to be done unfortunately.
There is a way around all this. Don't watch a movie about a book you have read or the opposite. Personally, I like all movies good or bad. I just take them as lighthearted short escapes from reality. I'm put off by vulgar language that is used for no good reason.