I am a good guesser and here's my guess: It's consumer driven marketing. If you put the fuel line into the package you can advertise a complete system. Plus the manufacturer gets to pick the hose, suitable for its use. Then there's the hassle of stringing a line. Hardline isn't that easy for a first-timer. Sure enough Cleetus lays the hardline on an exhaust pipe or has it balancing on an edge where it wears through a lot faster than rubber. Then again it's called a "bolt-on kit" and no way is hanging hardline bolt-on.
Take your pick. Notice that none of my guesses involve technical issues. A few lawsuits can eat the profits from the entire model line.

My '07 Ford pickup has an oil pressure gauge and a pressure SWITCH. No sensor or sender. An actual decision was made at Ford to use a gauge like a warning light. As soon as the engine starts the needle goes to 60% of full scale and sits there until the engine stops running. They'd rather pay for a few lunched engines than have every Tom, Dick and Betty bringing the car back because "the oil pressure is too high or too low."

R.