Classic MacOS isn't flat design, there are hard borders all over the place and (by today's standards) subtle cues of dimension throughout.
Flat design gets bad press because when it is done poorly, usability suffers greatly. I love the iPhone—and I like many aspects of its current UI aesthetic—but the UX design is woeful in many places, with countless problems that arose when the dumped the "classic iOS" appearance.
Most critically is that unless you're an experienced user of iOS, it is no longer obvious what (or where) is tappable. Buttons used to be obvious through design, now they are just text on the screen, sometimes in a different colour. Sometimes they're on the bottom, sometimes they're top-left, sometimes top-right. This all used to make sense when there were title bars and arrow-shaped back buttons that visually reacted when touched; now it is an ambiguous mess.