Yes, I get that. But in Mesos, for instance, you could use overlay-networks to get one virtual IP per HAProxy cluster (i.e. 3 HAProxy instances). They are then round-robin'd on each server in the whole cluster so that you have a transparent LB with a singular IP.
Round-robin routing is one way that really doesn't cut it at the level of scale that Github and others operate at. Overlay networks also typically have no notion of load and how to send traffic to the hop with the least amount of load.
