Why didn't guilty spark or the monitor of installation 00, the Ark clean up the flood hive during Halo 3

Why didn't guilty spark or the monitor of installation 00, the Ark clean up the flood hive that had crashed onto the ark during Halo 3 with Sentinels that Master Chief visited to retrieve Cortana?

Was the priority to build a new ring 07b instead?

Don't sentinel manufacturing facilities have enough power to make enough?

Why didn't sentinels attack the gravemind lying under installation 05?

Guilty spark and the monitor 00 weren't corruputed by the gravemind to not attack them like 2401 was.

The primary role of the Halo Array is to completely annihilate all sentient, organic life in the universe. This is a catch-all tactic that is designed to destroy both the Flood, and any and all potential carriers of the Flood. The primary purpose of the defense arrays is to handle small scale threats, not to destroy the Flood totally.

The Ark is the facility that was used to created the Halo Arrays. This effectively made this facility a production facility only, as it was buried so deep in space that no one, not even the Flood were even aware of it. As for its Monitor 000 Tragic Solitude, there is no information as to why it did not directly appear in the game (only through the Terminals, scattered throughout the campaign), but the resulting battle caused it to fall to rampancy.

When the UNSC and Covenant arrive at The Ark, a new ring, Installation 8 is already in production, in order to replace the initial ring that was destroyed by Chief in Halo 1. Again, due to its role as a production facility, defenses were not the main priority.

As the Monitor of Installation 5 was corrupted by the Gravemind, it had effectively labelled the Flood as a non-threat, by the actions of the Gravemind. This is how they were able to take High Charity, giving them away to find the Ark.