It would be good for the sport overall if Galen were more open, but the fact is, Galen is not and does not need to be.
Galen is paid his money because of his results. #1 in the US at the marathon, and probably at the 10K too. Olympic medalist at multiple distances and able to compete internationally (podium) at the marathon and 10K. And he holds the AR at the 10K, and indoor 2 mile, 3000 and 5000. Based on those accomplishments, his base salary with Nike is extremely high. Could he make more money by putting himself out there and being a social media influencer, or endorse more products by putting himself out there? Yes. But he does not want to and thanks to his results/Nike contract, he does not need to either.
If people hate that, blame the financial system that the shoe companies have allowed to hold down our sport for way too long. (Yes, I appreciate that the shoe companies are in many cases, the only source of funding for the athletes, but they structure their contracts and apply the pressure on the IAAF in such ways to help keep it that way).
Those who share openly may be more extroverted. Some may also need to keep up that popularity and Q rating because that is what keeps them sponsored and earning an income. Unlike Galen, a strong majority of them are not winning or even competitive in international competitions.