Can't really contribute anything here, except for the obvious answer to your initial question:
One blade good, sturdy, reliable, dangerous, lasts a life time or longer.
multiple thin blades in a plastic cage: unreliable, clogs up like a melonfarmer in no time, get blunt quickly requiring replacement (sold separately), lasts anywhere from one shave to a week for most, a month for the most devious, cunning and OCD of end users. Costs not even a bic lighter and a disposable ball pen to produce, costs the consumer its weight in gold. Stupid ass end user can't easily dispose of him- or herself, disposes of not-quite-fully-disposable razor blades instead in a regular interval, providing for keeping demand high and steady.
That's why.