There are several threads on this topic in the "Forum Information and FAQ" section, which you can review if you wish. The short version is, even with the difficult process, there is still enough spam getting through to keep the mod team busy. If it were only the CAPTCHA, it would be much worse. The idea is to make sure that people are reading the blog and are truly interested before joining, not schilling for their own blogs, trolling, or only wanting to argue politics or some personal axe to grind.
Personally, I think the bigger problem is how easy it is to access a bank account.