Because factory workers don't wear uniforms that look like that, ever since they picked up a bad association circa 1945. Google it - you won't find any photos of factory workers dressed like that.
That said, he works at Midjourney.. so it's a weird blind spot, if it's a blind spot.
Those are horizontal stripes, not vertical.
Horizontal Stripes were common in US-Prisons until the early 20th Century [1]. While vertical Stripes were common in Nazi-Camps. It's a very distinct difference, and I would think the source-material has more data about the horizontal than the vertical stripes. So it's fair to ask which specific prompt they had to get pictures from Nazi-labour-camps.