Why do junior faculty serve on PhD admissions committees at American universities? I only have knowledge of American universities:  I know of junior faculty that serve on graduate admissions committees in the STEM fields -- at top schools such as Harvard.  The things that I hear from them are quite interesting, for instance, how strong the recommendation letters are from the undergraduate applicants at top schools - that virtually all letters say that those students are the "best" that those professors have ever had.  But why do junior faculty such as "Assistant Professors" or "Titled Instructor" serve on graduate admissions committees? To me, the admissions process for graduate STEM programs are so important that I feel the committee should consist of senior faculty instead.
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.