They throttle, because there is much more demand than available bandwidth. However in cellular networks, you can add more bandwidth relatively cheaply by adding more nodes in high demand areas, and the total bandwidth of a single node is already higher than the promised 600 Mbps of Starlink. Therefore Starlink is not a competitor to cellular networks, because it can't compete by throughout nor latency. It competes with coverage though, but I'm not sure of there are enough rich people in boats to cover the investment.