Ah, but people really hate filling in forms. So much so that if you stick a complicated form in front of something they want to do, many of them will decide they don't want to do that thing so much anymore.
Even something as simple as asking for an email address in addition to username/password will lose you a percentage of signups. (As a real-world example, we tried removing all the fields from the signup page on Twiddla and saw a 1000% increase in users trying us out).
This form design seems to be a way of convincing people that they're not looking at a form, and therefore tricking them into filling it in to go do the thing they wanted to do in the first place.