For example why would they get star struck by a celebrity but not say a random person who teaches maths on youtube who has thousands of hits so could still be classified as 'well known' but not famous/ a celebrity?
I think because they feel they know them... And because there was so much marketing around those celebrities that they've made their lives so distorted and public, you can't help but feel you have something in common with them - whichever celebrity that might be you believe you 'click' with.
The same reason shit music is popular. It's not about who you are or how talented you are, it's about how pretty and marketable you are. If a Kardashian takes a dump people will trumpet it as significant, while geniuses are largely ignored. That's humanity for you.