They are part of Star Alliance so they most likely codeshare with United on their US routes. Presumably lack of demand is why AAL and DAL don’t fly there.

That is very odd. Their are daily flights between Warsaw and Toronto. LOT services daily while AIR CANADA or AIR CANADA ROUGE may pick up extra flights to Poland. Is it possible that this is just a hangover for American carriers from the cold war. There is an enormous Polish American community in the Midwest. I would pay attention to this valuable market.

Probably for the same reason no carriers service Cabo Verde - lack of interest. LOT has always been the flag carrier.

I once saw a LOT Il-62 parked on the ramp at ORD back in the mid '70s. Must have been a charter. Unfortunately, I did not have my camera with me.