People are happy and healthy in a place for all kinds of reasons. That doesn't make them a desirable settlement destination for others.
Maybe a town is incredibly racist and who lives there is all from the same longstanding community and they are all extremely close and have good personal relationships with each other and feel a strong sense of belonging to the place and the people. Maybe the town is perfectly suited to being students, young and single and there's a lot of things that cater to them. Maybe the town is better suited to families, couples, people who own dogs. Such people will gravitate there. This could mean they are all happy and healthy, but it would not mean that if I, some random bloke from a different state/culture/language/country moved there and tried to settle in and live my life, I would be particularly happy or healthy.
So yes, a place can be the happiest place in the world for the people who already live there, and still be undesirable for the "looking for a nice place to settle" population at large. However, having a highly employable, high income industry, lots of motivated hard workers, a vibrant art scene, and good weather makes a place a lot more universally desirable, and it takes a helluva lot of negatives to offset that desirability (which the bay area is trying really hard to do).