what even determines rankings?

Looks and social calendar

What they affect is how the Greek houses relate to and feel about each other over shallow things like looks and past social history.
They make some girls cry for hours when their not asked back to XYZ or end up at ABC's lawn on bid day, others to gloat that they made the 'cut' because they're pretty enough for BHE or whoever. Some schools have suicide watches on girls in dorms when they go through rush because tiers are seen to define your college life, when really they mean nothing and are not set by any voting or survey.
They beat down the chapters with a 'not stellar' past and keep 'up' some that need to reflect on their chapter functions as a whole to better serve their membership beyond college years.
What it really does is hurt every Greek. They prove to non-Greeks that its all about the shallow things, not the sisterhood/brotherhood, philanthropy, and networking we all say is a major part of our chapter life.
Tiers only hurt us and make us think badly about our Greek row in one way or another.

Tiers help no one and hurt everyone.

Tiers need to go away.