Why do 2nd and 3rd gen get compared even though it's different eras?

I don't really understand the queastion. I mean it's just normal to compare group from different eras, as long as people understand some things are difference and it's hard to compare the music and numbers of sales etc.

Girls' Generation and Twice is the obvious comparsion to do because both was groups with a lot of members and very similar concept. Same as comparing 2NE1 and Blackpink. It's like copy and paste the concepts.