Because Mamamoo are near an inactive group for the last 2 years, received basically 0 promotion outside of Korea in their entire career and sell like 100k .
It's not disrespectful to but Mamamoo in the bracket they belong. They are an aging group with a comparatively small fandom based on all metrics and they just sold like 70k tickets in the US....yet brand new, shiny GGs that have albums over 1m in sales in a week, or even a day, and they can't tour anywhere like that? Even globally some of these groups can't get 70k never mind in just the US alone....but MMM can? A near 10 yr old group who've, as said, basically never been pushed or promoted outside of Korea at all?
As I said in my later post, using album sales to quantify fandom size isn't always reliable, and it is the least reliable metric to measure the demand for Mamamoo. Even when they first started the concert phase of their careers back in 2016, every venue sold out instantly, even when their album sales weren't anywhere near 100k per comeback back then. Plus you said "based on all metrics", but you've only mentioned album sales.
Also doing a flat comparison of sales across generational gaps is very questionable, as the landscape of the industry has changed dramatically, and as far as I know, nobody has a fool-proof formula to adjust for how much album sales have skyrocketed across the board for girl groups.
And just because Mamamoo never pushed outside Korea doesn't mean the demand wasn't always there. You're talking to someone who went to all of their US events but one, prior to their world tour. People have literally been waiting years to go see them in person outside of KCON.
That's why I think using them as the barometer to judge 4th gen girl groups by is very questionable.