Because you need experience. At least in my country, unpaid internships are the norm; and you won't be taught anything, or held to a lower standard. You just do the job that you'd be normally paid for, just for free.

And even then, I had a few friends that had to go for another few months of unpaid job (not internship) later on. Because nobody would hire them without experience, and some companies don't consider internship to be one.