By default, when you use Surface>Revolve or Surface>Loft, output will be a NURBS surface.
But in your case, probably you have change the default setup on Output section, instead of a NURBS you could set this tool to output as polygons.
You can reset the tools back to default by resetting it.
If you are sure that you are outputting as NURBS, you might need to check whether you are in object mode when you issue convert command.
You might also want to check it through the Attribute Editor to see the type of your surface.