This is where EXO comes in.
A lot of people like to discredit them but it's truth that they ushered in a new era of physical sales in the Kpop market and thus why they are considered the start of the third generation.
1st gen Kpop groups existed during a time where physical album sales were the norm so selling a Million albums was very much achievable domestically.
During second 2nd gen, physical album sales were on a huge decline as digital downloads and streaming was becoming the norm. It was during this Era that YG Entertainment and their idols like Big Bang and 2NE1 were able to claim titles such as Kings and Queens of Kpop because they dominated the digital market and digitals were now considered the measurement of success. At this point physicals were more or less irrelevant and no one really paid much attention to it because no one was expected to sell more than like 50-100k.
Then came along EXO, shocking everyone when they started making waves in physical album sales. I remember back then there were lots of disagreements about who was most successful between V.I.Ps and EXOLs (when EXO was unmatched and at the peak of their careers). They helped paved a path internationally especially in China when it came to album sales. People began to care about selling lots of albums in order for their faves to be the best and the competition generated a huge rise in physical sales.
Then of course, later came along BTS that went even further by opening up the Western Market and having even greater albums sales that have yet to be matched. Now with social media and fans constantly competing to one-up each other, it's no surprise that 3rd gen has such high album sales when they are all now competing on a global scale.