For ages, girls have made their own clothing for their dolls. Massified dolls (read Barbie, Bratz, etc) generally use a razors and blades marketing strategy: sell the doll for a decent price and then make money with the accesories. Think of these as videogame expansion packs: cheaper to produce, easy to market because they draw on the brand's success and very lucrative. I took this picture today at Montevideo's flea market: they were selling dozens of knitted little costumes for Barbies (actually, the dolls on display were knock-offs. Barbies are way too expensive for the local standards).
By the way, you'll notice that I've become VERY interested in toys lately. Actually, my dissertation extensively deals with them. Certainly, unplugged games are a great way to learn game design, I don't contest that. However, good toys are even more elegant because they need to summarize one topic or one idea into a very simple system. If games are films, then toys are photos: a snapshot of play design.