They'd be stupid to throw away money / working components. Often, old PCs go back to a "pool", where they are upgraded or scavenged for parts, and then given to new employees. Only when they are too old to upgrade, or it is not worth the work, they are sold to employees or dumped. From the perspective of employees, you just get a new PC.

Essentially you just need a room for parts and a few employees part time, and you can use your computing resources much more efficiently - basically free money. Granted, it only works for organisations of a certain size (large corporations, universities, gov.).