In this specific case it’s about margins. They sell the razors (and starter packs) at cost then make all the money selling high margin blades. Quantity of engineering isn’t relevant, they’ve got their production lines well and truly set up, now they just hit “run” and "overcharge" you. That’s why dollar shave club exists. Even Harry’s bought an existing maker of blades in Germany.

This is evidenced by the fact that safety razors sell in 100-packs for $15 on Amazon ($0.15 each) while a 15-pack of Mach 3 cartridges (the simplest/oldest variety AFAIK) sells on Amazon for $22 ($1.45 each). Unless you're suggesting that the "engineering" it took for them to sell you 1.5 safety razors in a piece of plastic is worth 10X the price of the blades alone, I'd posit that's where the margin is.