The easy answer is that because we are a service sorority, and there is no "reason" to have houses. Most of our money goes towards service programming, and there is no infrastructure for things like dues and such to go towards housing. There is no Housing Foundation, no existing arm of Nationals to guide chapters reagarding housing.
You need bylaws, laws, regulations... it's a LOT of work. You also need a LOT Of insurance.
This is one of those areas where the differences between service and social sororities come out. YES you can make amazing friendships, YES you are working towards the same goal, but a service org is NOT the same as a social org. We exist for different reasons.
Now, that does not stop you and your sisters from living together. I roomed with two sisters off-campus for two years. We were about 1/4 mile from campus, and we ended up having a lot of dinners and stuff at the apartment. Ritual was held in an on-campus conference room.
I hope that my example was helpful. If housing is something that you feel your organization might benefit from, you should introduce it at the chapter and regional level, and prepare some sort of movement at your national convention.
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