Human beings rely on signals to communicate efficiently, especially in larger groups.
In the absence of well defined signals people will use what they have on hand and the results will be arbitrary at best and unhelpful in all likelihood due to propensity for misinterpretation and misrepresentation, especially since they may rely on inegalitarian mechanisms like gender or hair style or age or other intrinsic characteristics which may be gamed or are inaccessible to all.
By dismissing corporate culture so egregiously we have also done away with most established signalling mechanisms without offering any new substitutes.
The definition of dressing up in this context is in reference to establishing and manifesting visible and unambiguous signalling cues for such things as rank, status, team membership, privilege or clearance and other relevant metrics.
Since we're so sensitive to traditional methods it's difficult to convince anyone to put on a shirt and tie and quit acting like children but nobody has offered any better alternative than a printed T which in my professional experience and opinion is an all-too-naive solution that fails the military grade requirements after which we've modelled our private corporations.
Imagine you're a new member on a team looking around the floor for someone with an admin override. You need to ask six people before you get referred to Joe who has the keys to production. If Joe had a stripe on his jacket you'd be able to tell right away.