Business thrives on being passive, ironically. The more non-confrontational a face you can put on business, the more approachable it is by the masses. This applies to piercings, tattoos, religious memorabilia, t-shirt slogans, choice of dress, whether or not you swear, how tall, short, pretty, ugly you are. All of these things have a chance to either attract or repel people from a business.
So, if a business has a choice between an applicant that seems very passive in look, nothing challenging or out of the ordinary about the way they dress, the better candidate they will be for a job in that respect. You can't control for people's perception or social baggage in any regard, so all you can do is provide as neutral of a front as you possibly can.
In this sense, professional dress seems to stem from cultural norms of not being 'offensive' or 'challenging'. Whatever your culture deems 'neutral' is likely going to be synonymous with professional.
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