Efficiency and cost are components. Most large occupancy restrooms I’ve used, have a stall and urinal count that is roughly 3:1 to the sink count and a roughly 3:1 count of sinks to hand driers. There is typically only one door that leads to it also.
Then when the square footage of walls are compared between, say, 10 individual restrooms and 2 large ones. It all adds up to a significant cost increase for the individual ones as compared to the two large ones.
Also, the floor space required for the individual ones would be greater.
Granted, from an overall scheme, the above aren’t all that significant of a cost increase for an entire building. I for one would rather have many individual restrooms then the large multiple person ones.