Switch "health care provider" (e.g. physician, nurse, dietitian) for "marketer" and you can see the similarities between the 2 scenarios. The healthcare provider has collated the patient's information (e.g. BMI > 30, labs look like pre-diabetes, blood pressure risky), provided the health assessment and suggested next steps (i.e. lose weight sensibly, join this program, consider surgery etc.). And yet, the patient ignores the "sales pitch" and goes with the other company. In this scenario, the "other company" might be trying some risky quick diet regimen, drug or ignoring the advice altogether.
The marketer's frustrations is very similar to the healthcare provider's frustrations and leads both to consider the rationality of the customer/patient. Afterall, how could a person not make changes in light of all the convincing evidence presented? As Seth Godin puts it:
The problem is that your prospect doesn't care about any of those things. He cares about his boss or the story you're telling or the risk or the hassle of making a change. He cares about who you know and what other people will think when he tells them what he's done after he buys from you.
The opportunity, then, is not to insist that your customers get more rational, but instead to embrace just how irrational they are. Give them what they need. Help them satisfy their needs at the same time they get the measurable, rational results your product can give them in the long run.The obesity-related conversation can switch to one based on developing understanding and rapport. It might be an opportunity to explore ambivalence to change or eliciting change talk. Getting frustrated or even worse blaming the patient likely will lead to a stalemate. Your patients need you to help them change not to challenge their rationality.