Well, look at the graphic here: https://i.imgur.com/WdOZcNF.jpg https://i.imgur.com/WdOZcNF.jpg Parts in green are identical across variants. Parts that are si

Well, look at the graphic here: https://i.imgur.com/WdOZcNF.jpg https://i.imgur.com/WdOZcNF.jpg Parts in green are identical across variants. Parts that are similar but require slight changes from one variant to another are called "cousin" parts and are shown in blue. Unique parts are shown in purple. It was originally intended that the three variants share at least 75% of their parts in common. That required a modular kind of construction. That didn't work out so well. A modular construction requires "holes", right? But holes are heavy because you have to reinforce the structure around them to carry structural loads. Now retired test pilot Jon Beesley says this in this video: https://www.youtube.com/watch?v=RQ5gmZ74YBY https://www.youtube.com/watch?v=RQ5gmZ74YBY at 2:57. It's actually less costly, weight-wise, to build the aircraft structure without holes. For background, the aircraft that he's standing in front of is AA-1, which is the first F-35 aircraft. It represented a design that was 5000 pounds or so overweight. Many changes were done before the next F-35 to roll out, resulting in the current overall config. So in reality we are probably doing much of what you suggested, even though that was not the original intention. They share engines (well, the STOVL variant engine is a different part number because it has to interface with the lift fan, but it's highly similar) and electronics.