Various reasons. In Halo CE, Linda was clinically dead, and Fred and Kelly were on Reach. In Halo 2, during the Battle of Earth, they were undergoing psych evaluations and debriefings (much to the dismay of Lord Hood). In Halo 3, they were on Onyx. In Halo 4 they were conducting classified operations on the edge of UEG territory, removing pockets of the Covenant War still being fought.