There are a lot of government jobs I know of that are absolutely air-gapped. Your computer has basically no internet access, everything is stored on-prem. Hedge funds also tend to be extremely locked down, from what I saw when I interviewed. With certain data sets either having strict encryption-in-transit or a being stored in a quirky on-prem service. I can't imagine they're going to be dumping their data into Claude, etc.

As to why Windows is so dominant, I'm as clueless as you.