Here is the story answer (Destiny)

by Earendil, Wednesday, May 13, 2015, 19:39 (3736 days ago) @ MacAddictXIV

Well, at least parallels in the story to other known stories.

Guardians are an army in the same way Jedi are an army. In all the stories of Guardians long gone, there have been few times when more than 3-4 Guardians hunted together. At one point hundreds of guardians fought at once, and it was an epic disaster.

The theme appears to be that for everything the Tower* wants to do, a single guardian is enough to get it done. Stealth and quick strikes are the key, not heavy equipment that the enemy can see coming, thus no emphasis on building up equipment. We don't have tanks, because in general the feeling is if an enemy tank shoots at us we've already screwed up the mission.

*Tower: A note here. It is implied by more than one other entity that we represent the pansies of the solar system. Our no-time-to-explain friend has a large ship, and presumably guns on that ship. The Awoken also have an armed armada that is enough to keep the other races from bothering with them. The last safe city has defensive guns, but isn't bothering with offense. And in fact, the story makes it pretty clear that our character is taking some of the most offensive action that the tower has seen in a long time. Perhaps if this offensive attitude keeps up, earth may start building offensive weapons. But since the last safe city was in fact the last safe city, there appears to have been no reason or desire to build or use offensive things.