The majority of them are teenagers without a well-developed sense of ethics as yet. Many of them don't realize that they're putting themselves into legal hot water -- and the very fact that they don't actually understand their tools or targets makes them a lot easier to catch than an actual hacker.
Why do they do it? Because they're bored, because "hacking is cool", because they do not have acquired respect for others' space. They do it mostly for bragging rights because due to their lack of knowledge they don't actually gain anything much once they have broken into a system -- often they don't even know how to get around once they're in. And if they do, they'll delight in controlling your mouse or CD-tray, stealing random files, defacing your website. They are not usually financially motivated, but some of their antics result in lots of work for people who have to clean up after them, which can run up considerable costs for a business.
Friends don't let friends be script kiddies.