Stair Question

Prompted by a customer’s question, a veteran builder digs into a tried-and-true construction method in this query at the JLC Forums. Why, he wondered, do we buy large pieces of lumber for stair stringers and then cut so much of them away to produce the familiar stepped pattern? Wouldn’t it make more sense to leave the full width of the stringer and use metal supports under each step? That’s a detail his customer had noticed on interior and exterior stairs at a condo he stayed at not long ago, and the builder isn’t sure how to respond.