Because GGs are more prone to disbanding.
I'm only gonna mention the prominent ones. In 2016 we lost several groups that debut in 2009, namely groups like 2NE1/4Minute/Kara, 2017 miss A/Sistar. BEG technically didn't disband but are like on super long ass hiatus. T-ara technically didn't disband, there are rumors that they are gonna make a special comeback or something but .... the group kinda ended sadly. SM never disbands their groups so SNSD and f(x) are on indefinite hiatus. EXID is on indefinite hiatus as well. Girls Day is Girls Day.
The more recent one is actually back in 2019 when AOA made a comeback after Queendom and when you thought they will at least go on for a few more years, shit happened so the group is now on permanent silence because speaking out or literally showing their existence in the industry Mina might be unstable.
Then we have finally the one which lasted up till the most recent year in 2020, Apink.
I don't know if I miss out any group but I think Apink is the last and final group of the 2nd Gen that releases music. Naeun left the company but Apink is technically still 5. We don't know if they might release anything this year. So I guess the last standing 2nd Gen GG goes to Apink?
If we really really want to do so and include Brave Girls they technically are the last standing 2nd Gen GG but then again their current lineup is more of a "3rd Gen lineup"
With 3rd Gen starting in 2014 for GGs for groups such as Mamamoo/Red Velvet onwards, we are reaching the dreaded 7th year mark. Few more yeas later 2015 and 2016 debuts will start contract negotiations as well. We already lost Gfriend
Which group do you think will be the last standing 3rd Gen GG? We are gonna exclude Brave Girls from this because they kinda almost are an anomaly to this discussion. We might even call them 4th Gen LOLLLLL because they literally got famous only this year.
My guess maybe ... Oh My Girl? Oh My Girl seems to still be on a good streak and I don't know if WM might want to let go of them so soon.