Maybe because they are only 1500 tons bigger than an AB, are based on the AB design, use a hull form from 40 years ago, aren’t exactly optimised for electric drive, are designed around a lot of Korean systems which the USN doesn’t use, are built to Korean not USN standards, and dozens of other reasons. The StGs are good ships, but they are not the be all and end all of destroyer design nor are they universally suitable for everyone.
The Koreans and the Japanese have taken the basic AB design and modified it to their own needs. The USN is more than capable of doing that, too - indeed they have with the various AB flights; done, of course, largely by Gibbs and Cox, who also had major involvement early in both the Japanese and Korean programs. I rather think they are big enough boys and girls to derive their own design for a ship which, if the ABs are any guide, might be in production for the next 30 years.
The fact that they have produced a couple of less than totally satisfactory designs, both compromised at least to some extent by weapon system concepts which didn’t actually work out all that well, doesn’t mean they are doomed to do that again.
As for NIH,surely FFG62 proves that is not an issue? Not that there are really any suitable LSC designs around for the USN to adapt except the Zummies; oh, and maybe that Chinese thing......
