I would guess that the reason is that Gillette (or whoever) puts worse blades in their previous-generation cartridges in order to convince you to upgrade. A single blade works just fine. There's really no reason for these ridiculous things.

And there's no reason for the goo strip, either, if you'd shave with real shaving cream instead of whipped alcohol.

The swiveling head is the only reasonable innovation that cartridge razors have ever had, ignoring the cartridge itself. And that (the ease of blade changing) was predated by the Schick injector system.

Just, again, to point out the incredible overcharging for these things, the "Gillette Fusion" cartridges have 6 blades on them. Six cutting edges on the blades I get cost 45¢ (three double-edged blades). That leaves at least $1.50 for the cartridge, and probably more. In fact, UK investigators believe that it costs Gillette about 5p to make a cartridge, and they sell at a 4750% markup.