^ because as long as there's hope they'll return there's also more fans.
if they announced that they're not returning they would lose more fans, as a group and as solo artists as well.
for example - you don't like certain member but you still read news and interviews about that member in hope that GA as a group will be mentioned...if you knew they were gone then you wouldn't bother following that member.
so that's why they keep saying they'll get back, it benefits them, it doesn't mean they'll really come back.