Usually because it being windows they would reformat every 6 months and usually start the installation forget to put the key in and come back 2 hours later realising it was still sat on the 'enter serial' page. (Or even better they'd never be able to find the manual with the key on)
The irony of it never failed to amuse me either. In an attempt to stop piracy it's like putting a plaster on a shotgun wound. All MS were ever doing was penalising law abiding citizens.
Oh I also laughed at the horrendously long install times, whereas even with my 75mhz Performa 5200 OS 8.5 only ever took 16 minutes from start to finish...windows was always at least an hour.