They don't work because a lot of spam these days is human generated and not bots or scripts. The only way to deal with it is to manually approve registrations (even then the odd one or two can still get through).
I'v seen this people doing the "thank you for your post" one and a few others. There are some security scripts out that there are quite useful that are not being used on Polycount if you click view source. Captcha code is totally useless, its so out dated and exploited that having it on your site just attracts scummy spam…