T-shirt obsession is really a universal techie thing. Everything from the latest hip startup to the most stodgy of conventions give out t-shirts, often sizes far too large for most people to wear.
I guess. T-shirts just seem like one of those hyper-cheap things that companies can give out with their logo. Not much different than pens or foam 'stress toys'.
I swore I wouldn't wear startup shirts anymore, but then a shirts.io guy handed me one at a conference and it was so absurdly meta (print your startup shirt using our cloud API!) that I want to wear it all the time