Shrikes and Ferrets are not fighters
The basic rationale was that a CLAC could deposit an entire LAC group in a star system to operate on its own, or possibly with the support of an ammunition ship, while said carrier goes haring off on its own (or to go home, pick up another group, and return to deploy it elsewhere). In addition, the RMN was designing for mass production, and it always intended the new LAC classes for independent deployment in a system defense role, as well as to operate with the fleet aboard carriers.
Again, there are not fighters (not, at least, in the RMN's view). They are sublight parasite warships, and they were designed from the outset to have deployment constraints which were as flexible as possible, including the ability to remain on station for extended periods, to be launched from as much as several days out from a target system to come in under stealth on ballistic courses, etc. We may see specialized designs which eschew those capabilities in the interest of packing the nastiest possible punch into the smallest possible package, but for now, the RMN prefers the flexibility of its basic design and will probably stick with it.
The RHN would probably like the same capabilities, but they were forced by their tech limitations to pursue an austere design whose less capable performance had to be tweaked to the maximum. Hence they have accepted a much shorter endurance, which could turn around and bite them on the butts in a scenario which sees a Manty LAC force dodging around (in an effort to get at system infrastructure, say) until the Havenite defenders run out of life support and have to give them the opening they want. Unlikely, true, but still a possibility.