I recently came across an article about fashion in Silicon Valley. It is interesting, but it doesn't really touch on why being fashionable, in the context of spending money on Chanel and other brands, doesn't work too well at start ups.
1. Fashion is a status game that most engineers are not good at. Making sure that engineers are appreciated is a big deal, so bringing in people who put a lot of importance on status outside of the ability to solve problems is not going to help the culture of a company.
2. The article mentions brands like Chanel. These things cost money. Spending lots of money around people who are taking low salary/high equity compensation is not great for the culture of a start up. It encourages people to leave in order to take high pay/low equity at a more mature technology company. A entrepreneur who always dresses in expensive clothing is showing off their wealth the same way as one who drives a Maserati.
3. Following fashion rules and trends is correlated with other types of conformity. In a culture that prides itself on how non-conformist they are an interest in fashion is not a good signal.
That said, engineers at start ups have been willing to push fashion boundaries in ways not seen elsewhere in the professional world. The difference is that this is done by breaking fashion rules, ignoring current trends and not spending lots of money at boutique shops. (If I was less respectful of my friends' privacy I would now post a picture of three engineers from a very successful start up: one person has a mustache, the other with a mullet and the third with a neck-beard) Maybe the fashion industry, like the car industry, can make more inroads into Silicon Valley at the successful companies where many people have already made their money but it's going to be hard for them to get in on the ground floor and attract people building the next great companies.