Which is what is so bizarre about it. Everything else about Apple is about cleanliness, focus, minimalism, simplicity.
Yet the windowing system is designed so that everything's overlapping, it's hard to pick out your foreground window from 20 other background windows of varying shapes and sizes and positions, and bringing up Expose/Mission Control shows you something like a detective's corkboard as he tries to connect the dots to solve the case of the serial killer.
It's quite jarring when the rest of the Apple aesthetic is clean minimal lines, and then your windows are this separate universe where it looks like a three-year-old tossed rectangles all over the place and never cleaned up after themselves.