Because in the end this is all about group conformity.

There are unspoken dress codes everywhere. in the City of London men have to wear black suits, red ties, and black leather shoes with laces and leather soles (or something like that). If they don't conform they don't progress precisely because they don't conform and failing to strive to conform is a character flaw in a corporate setting.

Birkenstocks are for lesbians works both ways - on the one hand if an American woman wants to buy sandals she'll carefully consider whether she wants to telegraph to the world that she's a DFH. Coversely, group conformity among American feminists requires adherence to a particular counter-cultural dress code.

En un viejo país ineficiente, algo así como España entre dos guerras civiles, poseer una casa y poca hacienda y memoria ninguna. -- Gil de Biedma