As you've seen, those user contributions are offered on a "I think these might be useful" basis. Validating a force field tends to mean showing that a set of relevant single-point energy calculations work the same way in the original implementation (ie in AMBER software) and the new one. Those might well relate to the set of calculations they did when parameterizing, so someone needs to find out what exists and port it to GROMACS formats, etc. People tend not to want to do that, so such force fields don't go into official GROMACS distributions, because if they were, then everyone would assume such testing had happened.
You should already know whether 14SB was a complete re-work of all or part of the force field, so probably some unchanged files are to be expected.