Because maybe you did something interesting in your young life that others aspire to achieve like: front a popular rock band[0], star in a popular television series[1] or a few movies[2][3], or even change a professional sport [4]?
I'm not sure why you linked to these people though? These achievements may be tales to recite to your grandchildren but none of this should come up in academic entrance decisions. These people have chosen a career in entertainment which is completely orthogonal to higher learning.
Many universities aren't just about academics though. They wouldn't have things like, say, theatre clubs if they were. At some level, universities are about helping to prepare young people for life and a career. Certainly a lot of what I got out of the various degrees I obtained had very little to do with specific courses I took.
Yes, different clubs are common and beneficial, and there's a lot of growing taking place during your uni years. But the parent was linking to professional actors, and I fail to see any relation there. The topic wasn't about admissions to theatrical majors in art universities.
I would much rather learn from somebody who has done it professionally at a very high level than some academic (or worse, a wannabe academic) who is all theory and no results.
You see no value to taking a music theory class with a platinum artist in your discussions on song structure? Or maybe the rock star wants to expand his horizons a bit and is also in your Ethics class discussing the effect of Napster on his ability to afford tuition. Or maybe he sits next to you in Econ and wants to use his experience in the music biz as the basis for your group project. etc, etc...
Forget the exam nerds, I'll take the real players every time. YMMV
I'm all for rock stars expanding their horizons if they score better than those nerds. Though the education is even in that case likely better invested in the student aiming to get a career in the field, rather than in a rock star's recreational studies.
The "real players" could also visit as guest lecturers on your music theory class, assuming they have good enough merits on the subject.
At least a couple of them were English Lit majors. But I don't really disagree that well-known actors who attended university off and on are really the best examples.
That said, you might be surprised at the number of people who even go to a school like MIT and end up in journalism or something else that isn't obviously related to what you associate the school with.