This is a really wide open question. There are theories, but no one has any truly strong evidence. Steven Pinker is an evolutionary psychologist and linguist, and wrote an excellent book on the subject. It goes into several of the more common theories, and then lays out his own theory. Much of the book may be fairly dry if you don't have a real interest in language itself (how language works, as opposed to how it evolved), but I found it very worthwhile and fascinating.
Oh, are we quibbling? I see from the many reviews that RS posts on their site that it's actually "learn like a baby".
Of course, they've put a testimonial in one of their ]case studies](http://www.google.com/url?sa=t&rct=j&q=site%3Arosettastone.com%20learn%20like%20a%20baby%20-reviews&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Fresources.rosettastone.com%2FCDN%2Fus%2Fpdfs%2FCase_Study_Defence_Academy_re...) that actually does read "learn like a child".
The important this here is... I don't need to back it up. Look through the many comments all over the internet complaining about the product. Or you could read a critical analysis in this rather popular book. Or this one.
Oh whatever, I'm probably wrong. After all, there is a veritable army of fluent polyglots out there who rely on Rosetta Stone products... right???