Indeed. There's a reason for that: Most modern non-fiction books are the result of a publisher giving a book deal to an author to expand an article they wrote and people who are going to spend $20 on a book want to feel like they are getting their money's worth (more pages == better deal).

Few modern authors are willing to buck the trend and publish a 100 page book, but they do exist. Take a look at Christopher Alexander's Notes on the Synthesis of Form.

I think you're reading the wrong books, then. I wish the books I read were 100 times longer.

That depends entirely on the quality of the writing.