I'm sure they considered calling them guilds, and perhaps using the word "guild" would be a more accurate term for them, but Bungie is trying hard to find the balance between fantasy and sci-fi, and you can see that in a lot of things they make. Tigerman is a good example of too much fantasy, and the original idea of living inside an alien space-sphere started to apply too much sci-fi. Avoiding the term "guilds" is a part of that entertaining balancing act