Maybe because they recently had a recruiting event and handed out shirts?
I got a tshirt at the techto meetup here in Toronto a few months ago.
Palantir T-shirts are super convenient. I don't care at all that someone else is wearing the same t-shirt, if it means I don't have to go clothes shopping.
Palantir tshirts are very popular on Australian university campuses too in my experience, from recruiting events they run.