If you have to pay $7 per month for each computer, and you have 100,000 computers, you will be paying $700,000 per month. Multiply that by 12, and you get $8,400,000 per year. So, for large businesses, the likes of which Windows 10 Enterprise is targeted towards, that can add up rapidly. IDK how exactly how many computers a given enterprise might have, but 100,000 doesn't seem too far out of the ballpark.
IDK why people would be willing to pay so much for software. After all, there's almost always similar programs available for $0. Operating Systems are no exception. While Windows 10 might cost millions each year, Several GNU/Linux distros cost a few hundred a year, while the majority of distros don't cost a thing.