Well, for one thing law school is usually more expensive than undergrad. For another, it's much more miserable - most people don't want to stay longer than three years. Third, as mentioned before, the curriculum options are much more limited. Also, employers don't care if you stay in law school an extra semester just to take a few more classes. In undergrad, those extra language or accounting classes in a 9th semester might make a difference in the job market.
Many students do externships so they can graduate a semester early. I think that most people finish in three years except for folks who are doing dual degrees. I don't have statistics like other folks, but I've only met two people who were in law school for four years, they both did dual degrees and they both expressed that they knew very, very few people who stayed longer than three years.