@cassa333
Because a big part of the problem is people traffickers and those who smuggle themselves in trucks. (They could be arriving by container ship, too but I know nothing of dock operations and cannot prove any such route exists).
Another possibility is benefit fraudsters. Someone on AB posted a news story where a flat was used by one 'organiser' who lived there permananently and up to five other claimants - a fixed set of name/NI numbers getting housing cheques etc but, in reality, a succession of people passing through the property, using the address as a base while being part of the organiser's work gang. They could return home without troubling us for an NI number at all!
The only scenario I can think of which would 'consume' NI numbers would be for seasonal workers to be making repeated visits, each, with a new false name every year, or every trip. Other than a desire to escape detection by Interpol and/or their country of origin, I can think of no reason for doing this. It may be a response to some beaurocratic hurdle, like time limits on work visa re-issues (eg max of 'n' times in every 'p' years, say. I plead ignorance on foreign travel & working abroad issues.
Still, it amazes me how much of peculiar human behaviour is merely a response to bureacracy.
Just don't ask any tax haven about their raison d'être. ;-)