Why is IP (intellectual property) law necessary?

IP law (intellectual property - copyright/patent/trademark) assumes that artists and inventors will not have an incentive to create unless they have a monopoly on their invention or creative work for a specific point of time. Is that true? Let's look at history... Shakespeare wrote his works without copyright protection. The Mona Lisa was painted without copyright protection. The Guttenberg printing press was invented without patent protection. According to those who claim there's no incentive to create such works...this all should have beeen impossible. Maybe human beings are not as purely profit driven as the economists say. After all, this blog is not being driven by profit motives. If IP law is only dubiously necessary to promote creative and scientific works...then it may be unconstitutional. Why is it so accepted that it is such a necessity? Thoughts/comments on IP law?