[CapricornNoble]

>>>I bet half of it is plastic ("polyester") unhealthy and damaging to the environment in 100 different ways.

You don't have to guess at its contents, you could just....read the linked product description. It's mostly Merino wool, which is a natural fiber. Para-Aramid is synthetic but isn't a proven carcinogen.

We stopped using polyester moisture-wicking shirts when we learned that the fabric melts into your flesh during IED attacks. Cotton has its uses, but it is not as durable nor as performant without at least a blended fabric or other production compromises (such as overall additional weight).

None of which changes my original point: you can't just buy a sewing machine, get a bolt of cloth, and fabricate a shirt of equivalent performance in your own home.