Whenever books are translated to film, there is almost always something lost in this way.
The Coens are definitely not lazy and it presents a great challenge to tell the No Country For Old Men story without hitting the audience over the head with constant voiceovers explaining the action.
It is hard to convey the thoughts and feelings of a character without their inner dialog that is easy to do in books.
On film, doing something very hacky like constant voiceover monologues and exposition too often serves as a lazy way for filmmakers to fit these kinds of thoughts in films.