This makes it hard to estimate the need for lounge places and other amenities like business class checkin and security lines.

If the lounge access comes with business class ticket irrespective of status, the airline can simply look up their seatmap, make the schedule for the airport and they know the exact maximum number of people who are entitled to be in the lounge, and refurbish/rebuild the lounge accordingly.

If the airline goes around selling lounge memberships for cash or handing them out to frequent flyer elites, they never know how many paid lounge members or elite level members may want to fly in coach of the same flight or on the same day.