This one isn't rocket science.
Framework isn't as mass market as your typical laptop competitors. You can make an assumption a purchaser of framework is tech minded / informed shopper.
You can also see their prebuilt option is Windows only.
When going DIY, adding Windows is between $225 and $339 depending on edition.
Most informed shoppers and techheads also know you can go to key selling websites and get completely legal and working Windows keys for <$20.
Why would people volunteer to pay 1000% more?