Makes me think of “upper bay” style decoys which are usually turned on a lathe, in this case, a round bottom seems necessary just to make them. They generally have some lead on the bottom, but no keel.
They bob side to side a good bit, but probably self-right much easier than a solid body flat bottom decoy using the same amount of weight.
Probably a similar situation with carolina style decoys
For more contemporary non-upper bay, non-Carolina etc. style decoys, I can really only guess. But real ducks are pretty round, so I can see why some would like it.