Because if are a nobody and noone recognizes your face people will heuristically assume you aren't a "down to earth billionaire", but are instead just a poor, average person and thus insignificant. Zuck can appear to be humble in his clothes, because everyone knows who Zuck is and people expect famous billionaires to be smug assholes, so this is a way to countersignal and pretend to be "just like one of you, guys". Since you are "just one of us", it's in your interest to appear as something more.