Rankings matter because rankings matter to people. If we lived in a purely meritocratic world I would agree with you that for a lot of degrees it wouldn't matter where you go. But the rankings are confounder for two important aspects: money and network reach. Money means bigger and more advanced labs (comparatively. I've been to big school labs and small school labs. Both are hacked together but there's sure a difference). Networking helps you get more papers published, more prestigious internships (i.e. more on job training), more internships in general (lots of students get denied in tighter job markets where it is easy for top tier uni students despite similar skill levels at Freshman and Sophomore years. But internships beget internships), and the networking gets you into the door when you graduate.

I strongly agree that it doesn't matter much where you go to get your education if your goal is to get educated. But if in addition to that education you want a promising career, rankings do matter.