Except that one isn’t as good for a general greeting as it means “each of you individually”

“Y’all suck” - the group of you sucks

“All’a y’all suck” - each of you individually sucks, there is not a one of you who doesn’t, the group also sucking is implied

A general greeting is generally addressed to the crowd, with collective individual statements being more intimate and often only used during more emotional statements. Eg, “all y’all have contributed to the success that brought us here tonight, thank you” is normal, but “all y’all’ve been an excellent audience tonight” ain’t right