The funny thing is that we actually have a quite big commercial market for such utility vehicles, currently met by dropside vans: vehicles built on a van frame with a flatbed or other custom equipment on the back.

This is not really that different from how the F-series works either, providing a frame where the rear can be built up dependent on needs.

It's just that the F-150 is a terrible fit. Our dropside vans are small with a very large footprint-to-bed ratio, is light and have low fuel consumption, is almost always custom, and has zero luxury or otherwise private appeal. There is no demand for e.g. engine performance, as the light vehicle class has load and towing limits that are easily met.

The F-150 is too bulky for casual city use and without a need to optimize have a low footprint-to-bed ratio, is heavy with and thirsty, is often just used "as is" with the stock bed, and is often used privately with a significant "muscle" appeal. In order to be driven with a regular license and not be affected by truck speed limits, tracking requirements and driving/rest time limits, it would still be registered as a light vehicle, making any additional load bearing/towing capacity unusuable.

There's nothing wrong with models being extremely optimized for specific markets and unfit for others. I imagine there aren't many EU-style dropside vans in the US, not to mention EU-style trucks. Japanese Kei cars are an even more extreme case of such market optimization.