Capitalism. Some people want to buy them, so some people make and sell them. It's not that hard to figure out. If there was no market for them, they probably wouldn't be very many of them made.
(This is not a commentary on whether it's "right" or "wrong", or on the taste or judgment of any individuals involved. So don't interpret my response as if it is. It's simply intended as a response to the OP's question.)
My mistake. I passed through here earlier and quickly scanned. I thought the post was asking why *can't* you find such shirts and posted those links to counter the implication (after a quick google search).
I should have read more carefully if I was going to bother commenting.