I think at Alabama the membership of the sororities is more likely to skew in-state, though. And being from certain high schools is definitely helpful in joining certain chapters-I know that's true at Clemson too. It's not even that in-state PNMs are somehow "better" but that women already in the chapters are more likely to know them and thus more likely to advocate for them than women who aren't already known.
In my experience, the sought-after PNMs weren't all from South Carolina but were very much all from the South, and there was a very strong North v. South divide among the fraternities and sororities, i.e. certain groups were "northern" and certain groups were "southern" because of where most of their members came from. Not sure if this is true at other similar schools.
At Alabama, the split on PNMs is about 50/50 in state vs oos. Of course, some chapters are OOS heavy and vice versa. But since they place over 90%,obviously OOS doesn't matter...as long as you keep an open mind
The smaller liberal arts colleges that pull a diverse population across states. The schools and chapters might be small, but the presence of greek life on those campuses can be intense -- especially if a school is "in the middle of nowhere." But those schools aren't so worried about high school connections, it's likely those connections don't exist enough to make a dent in recruitment.