Why do junior faculty serve on PhD admissions committees at American universities?
If junior faculty members aren't allowed to sit on an admissions committee, then  by the time they become senior faculty they won't have had any experience of sitting on admissions committees. The junior members need to learn how to become senior members and this is just one part of it!

I also don't think that graduate admissions decisions are quite so monumentally important (or tricky) as you imply; for top institutions such as Harvard you will already have a self-selected pool of very good appplicants to choose from, and I imagine it's difficult to make a bad pick.