I think some people might buy them (even if of average intelligence) because as a complete beginner they may not have the confidence to know that they are able to make one themselves.
You might extend the argument to a lot of what woodworkers do - is there anything (power tools aside) woodworkers use that could not be manufactured at home? Steel billets, I grant you, might be a tad difficult, but plane blades? Check. Saws? Check. Chisels? At a push, check. Straight edge? With a little effort, probably. Winding sticks? Yep. Shooting board? Check. Mallet? Check. Bench? Many threads on the subject. You see what Im getting at?
Perhaps a bench hook is about the easiest 'tool' to make (apart from a push stick or clamping caul), but that doesnt mean it's easier or quicker (see post above) to do so.
Cheers,
Adam