One common reason I heard is that bankers simply like to flex - not necessarily their visible arms, but where they work - as many firms give out branded vests. "It's an earned status thing. You put in a lot of work to get to equity research for your team, to get to the PM role for a team, to be able to get to that said firm," one analyst told me. "That's why you wear it."
Our culture embodies mutually exclusive beliefs in tribalism and individual exceptionalism. Tribalism used to depend on family; over the past millennium, it increasingly has come to depend on wealth and status, with new elite groupings developing every day-now they're called "lifestyles." The economic stratification of America isn't dissimilar to the caste system of India, the main distinction being that Americans believe they have the opportunity to gain more and more status if they put their shoulders to the grindstones and employ good ol' fashioned industriousness.
Wearing a Loro Piana sweater on a yacht that's strategically anchored in international waters so as to avoid any criminal financial culpability. Organic blueberries. Buying organic blueberries from a weekday farmers' market. Buying organic blueberries from a weekday farmers' market after laying off half your staff via e-mail ("Sent from my iPhone"). A bottle of Aesop hand wash in your bathroom. A bottle of Aesop hand wash in your guest bathroom.