The Bright Republic learned a very painful truth after waging several costly battles against the sandman fleets that adopted the swarm configuration.
The Starfighter Corps was not a silver bullet.
Against the wrong configurations, the many weaknesses of starfighters became very pronounced.
Without costly exotics to fortify their armor systems, their mundane armor plating was barely better than that of a commercial shuttle!
Their designers cut so many corners in their design that their armor plating was only really good enough to resist spacedust, micrometeorites and the occasional space junk floating in deep space.
They fared poorly against high-volume firepower!
Not only that, but their starfighter pilots were hardly as professional and disciplined as actual soldiers.
By far, most starfighter pilots were young and poorly-educated citizens of the Bright Republic or the fallen border states.