It actually is an option from most sellers, it's just not really promoted, but if you look you can find it.
If it was easier to find people would remove windows thinking "why do I need this" - and then call tech support when the computer doesn't work.
A single tech support call eats all the profits of the sale.
I'm pricing new cars right now and the makers are doing these interactive build-your-car websites where you select every single accessory and option and watch the price go up or down accordingly. Admittedly, Dell has had such a service for years, but preinstalled Windows is usually non-negotiable. Just automate the OS install process (if it isn't already) and let the user shave the $80 off the price if he prefers a free OS rather than a paid OS.