Lot of saltiness against a company that pretty much single-handedly, against tough odds, has turned the automotive industry direction from fossil fuel power to electric; so much so that Mustang is going to be electric. Musk is a dork, attention-seeker, and a promise first and worry about delivery later fundraiser, but it was pretty much what was needed to win. He has been laughed at for everything from using "laptop" batteries in cars to getting into rocketry with PayPal background, but he usually has the last word.
This truck is unusual, but I can clearly see the engineering reasoning. Anyone here that has designed anything reasonably complicated (an Apple Watch, a VR headset, etc.) knows that the final product is governed largely by the philosophical approach chosen. For current cars and trucks, the primary philosophy is to fit the design to current manufacturing/supply chain ecosystem. Here, Musk has started with a blank slate and said, "what if I start with the cubic volume of an F-150 and nothing else?" His team reached the conclusion that by building an exoskeleton (think insect with hard shell), they can better use the space than the traditional body-on-frame skeleton approach (think mammals). Obviously, this results in a completely different vehicle. Now, the outside shell can just be a cuboid, or a rounded cylinder, or a pretty good aerodynamic wedge shape. Of course, the wedge shape can be smoothed, but secondary issues are many, such as Musk's love for whimsy vs traditional ID team's concept of what is appealing, the exoskeleton's strength in bending and torsion as a function of modifying the wedge shape, manufacturing ease/cost, etc. This is when UX team also gets involved regarding ease of use, story boarding a day with the product, etc.
I think, at the end, Musk must have said, "let me roll with a radical design, and we still have lot of time to massage everything about it." After all, Tesla still has Model Y to build and deliver before even seriously starting work on the truck.
TL,DR: Design is based on the chosen philosophy, and this truck is just Musk playing around with the final product most likely to be different enough while maintaining the same concept, imo.