Idk if they did have the firepower. Halo 2 and 3 reveal that total Covenant fleetpower was not as large as the UNSC believed at the time, and crucially *they* had little knowledge of UNSC naval capacity either. Probably made sense not to waste ships.

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Their de facto independent cities who have better harbors than Empire (who has a riverine fleet mostly) can put quite a lot of fleetpower together. And some coastal Barons also venerate Manann as much as the Lady, and serving on a deck as officer and marine isn't seen as dishonorable trade.