For a single traveller, a Roomette and a Cabin for 1 seem indeed to be equivalent. However, for 2 passengers willing to share a Roomette, I don’t see any reason why they would opt for two separate Cabins for 1, as Upper and Lower Berths are significantly cheaper, while a Cabin for 2 offers much more space for the same price as 2 Cabins for 1 (as you correctly noted)...
A "Cabin for 1" IS a roomette. They just renamed it to a more cruise ship-like name.
Amtrak's "roomettes" started life being called "Economy Bedrooms" and they retained that nomenclature for a long, long time. Then briefly they were "Bedrooms" (as opposed to "Deluxe Bedrooms") but that was confusing. Then they renamed them "roomettes", a nice traditional railroady term. They are, in fact an accommodation unique to Amtrak and are kind of a mash-up of a roomette and a section. In Superliners, they are essentially a section with much worse beds, a door and a wall.