They go to their doctors to know when to start medical interventions.
Even simple waist-to-hip ratio assessement would be a big win for identifying lifestyle concerns, but neither that nor the far more involved VO2Max and DEXA rests are going to become standard unless they becoming a gaiting condition for some celebrated drug or procedure.
And that's as much because of the patient's perspective of what they want from doctors as it is because of the doctors or broader establishment.
"I tell my patients they will need to lose weight, but whatever they do they must not lose it too fast. They always follow that advice just fine"
Settling for subpar "could be good enough" heuristics instead of doing actual measurements is partly why people are less receptive.
Basically it'd just be telling them they're fat, and not what's their actual individual risk, and it would also make tracking progress overly difficult: what if after a year of exercice their VO2Max greatly improves while waist to hip only slightly changed ?
Overweight people already have distrust of doctors just telling them to first lose weight and dismiss their specific requests for help, additional advice vaguely based on statistics would probably not help.
People don't want to hear it because they are out of shape.
One should stick to the facts however.
Waist-to-hip is a good predictor for generic subjects who otherwise never cared about improving their health. If the goal is to actually improve health, that assumption stops being true and you can't rely on generic heuristics anymore.
Frankly, the vast majority of people don't need fancy tests. The thing is, the healthcare system is not about optimizing health or performance. It's mostly about fixing problems and heading off the most common ones in the easiest ways. If the goal was to optimize, it would be relatively easy for a trained individual to examine most people and tell them to lose weight and exercise more. Hell, you could blindfold the person and their advice would still be right like 80% of the time.
At the very least, good medical care provides information. What the individual does with it is up to them.
Yes, medical care should provide information. There are lower tech and cheaper ways to provide actionable information. But like you said, it's up to the individual. Either the will is there or it isn't. If someone knows they would benefit from the visual representation of a scan, they can ask for one. Technically DEXA carries some risk with x-ray exposure if done on a recurring basis.
"Excuse me sir, I have you VO2Max and DEXA scan results. It shows your cardiovascular fitness and body fat is problematic. But we already knew that since your BMI is 35 and you told me you don't exercise at all".
Most people have these at home and fail to take meaningful action based on the results.