If they don’t have songwriting or production credits, most artists/musicians make the bulk of their money off tours, endorsements and merch, where they typically have a larger percentage, not album sales.
But within this specific discussion, there’s a potential chicken and egg component here - are people purchasing more albums because there are more members (i.e. there’s a member that suits every taste.)? So, by adding members, are they potentially increasing their profits, particularly in those more profitable areas of tours and merch, compared to a soloist? So the album is serving as the “loss leader” to get people interested in the real moneymaker. Since 2008, the top selling album in Korea has been a group, not a soloist.