because the content can be learned in situ, in books, online, through practice projects, with friends, &c. "The content" of a PhD is largely about stuff that's

because the content can be learned in situ, in books, online, through practice projects, with friends, &c. "The content" of a PhD is largely about stuff that's not yet in any books or papers (let alone tutorials and practice exercises). The role of the PhD student is to write them (after discovering that content).