Hello,
You mean like pre-installed software (like factory installed Windows XP) where the manufacturer don't provide you with a Windows XP installation CD?
Firstly I think the question that has been set is flawed, because it implies that all computer retailers don't include backups of installed software on a backup medium.
Most laptop and PC manufacturers do provide CDs containing software backups for end users in the form of OEM CD-ROMS or as a recovery CD, and if they don't, then most of the manufacturers provide a feature within the OS which allows the end user to create a set of backup / recovery CDs to restore the system to a factory state.
But to answer the question, the main reason I can think of is because most system vendors deploy OEM Windows XP operating systems (which are branded and customized to the manufacturer's preference). What they do is deploy a customized OEM version of Windows onto multiple systems at the factory, activate Windows at the factory, then distribute the system to retailers for consumers to purchase. System vendors can therefore save on costs in using OEM software, and save costs in having to produce and distribute recovery discs / backup software CDs to end users.
In turn, if the system manufacturer is saving in costs by deploying an OEM version of Windows, then the consumer too should also be benefiting - from a lower retail price of the cost of the system.