The domain knowledge + software engineer skills + computer graphics skills needed are extremely rare.
"It is in this context that a number of practices, who represent a revenue stream for Autodesk of over $22m over the last 5 years"
A $22m over 5 years is not a large deal for the large CAD companies such as Dassault, PTC, Siemens, Nemetschek.
Revit as a software package started as Reflex and then acquired by PTC, PTC decided they couldn't make enough money off it, and basically let former employees leave and start Charles River Software with the Reflex code base. The amount of work required to create Revit, It would not be good return on investment for any company other than Autodesk to sell it. The only reason it makes sense for Autodesk is it keeps the AutoCAD DWG monopoly going.