I've often thought the same. Execution is the hard part, especially with regard to conversions. It is really tough to get right, even with a mature product. And, given the sea of noise that invades everyone's lives, trying to be heard with a product presentation that is purposely subpar seems unrealistic.

So, one problem with lean is that you're actually testing two variables: the idea and the execution. When people don't convert, is it the idea, the copy, the design, something else? It may be that you just need to iterate on your copy/conversion funnel, etc. a dozen times. But, lean de-emphasizes the implementation; encouraging you to instead view the feedback as primarily relevant to your idea, and so move on in the face of low conversions.