This is a real problem -- most big brands now supply a factory image of the OS on a partition on the hard drive. Lord knows what you are supposed to do if the HD fails but in most other situations the user can rescue a corrupt installation simply by reinstalling from the image partition.
Personally I would not buy from a supplier who did not include an original CD/DVD of Windows and I would be prepared to pay a little (but not much) more for that.
It is arguable that supplying an actual disk increases the risk of piracy due to various fixes which allow one to install on several machines. But I guess that, above all, the industry likes the idea that when a computer crashes the owner may well simply buy a new one -- further sale of hardware, further sales of Windows (hurrah and to hell with the environment).