Where do genres come from?

We spend a lot of time discussing how genres work, where to draw the lines between genres, and which tropes and motifs are essential to a particular genre. But I haven't seen as much chatter about where genres come from and why they exist. If anything, I usually see people saying things like, "Genres are marketing categories.” Which is true, as far as it goes — except that it doesn't address why those marketing categories matter, or why they're so effective. So here's my working theory for why ...