It’s about equilibrium and supply and demand.
Covid related problems have caused a surge in demand. Should cause prices to raise and worker pay to increase which will cause an equilibrium at a different point.
But this surge is likely only temporary. If the system has an equilibrium point at two shifts (because of high longshore pay for that 3rd shift and thus more investment in port infrastructure) that actually gives the system more surge capacity than had the system settled at three shifts with fewer berths and cranes.
Far quicker, cheaper and easier to add a third shift than to add berths and cranes.
fwiw.