Military members have "no political statements" drilled into them. You will not get the Joint Chiefs or anyone else to make a statement that would be read as opposing the President, even on a matter as obvious as this. That's not aided by the fact that his party has a long-standing preference among the military in general -- far from universal, but it means that they are even less likely to want to break that injunction. They prefer to maintain a "neutral" position even though they know that their silence will be read as assent.