The problem is of course, that a low effort design prototype still looks and feels like a low effort design prototype. But the customer sees 5 variants of the low effort prototype and can decide which one is iterated on further.
If you program a low effort software prototype, it should still kinda work and this is where the customer would decide to run with it.
Also software (like graphic design) is a field where people without any skill tend to have strong opinions about how things should look or work, which makes manipulazing the customer into taking a decent solution part of the job.