Because the 100€ car comes pre-installed in their new garage.
If you buy a product preinstalled, well then you pay for that. Look for HP ZBook 17 G2 in your favourite shop. You could buy it for x money without OS (freeDOS) or for x +100 with Win preinstalled.
Most of people choose preinstalled Win even its more expensive. Most companies who has IT specialist so could support Linux specialty choose to pay. Why? Because owners of the companies find out Win is cheaper. Less TCO. Thats it.