True guardians are already dead (in theory). However each new guardian was brought back to life by their ghost , AKA Resurrected , therefore guardians are very much alive thanks to the light of the traveller. The darkness also has a similar thing, you can see how the hive use shards of the travellers light to store the souls of ancient hive in them, and corrupt them to the darkness. So if the light of the traveller can keep ancient hive alive for centuries then why cant it bring dead guardians back to life?