Because the boat is already there. It is being built on the landward side of the bridge and he wants to take it to sea.
The central section lifts up to allow ships to pass, but Bezos's ship is still too tall. The rail line on it is no longer used, so I don't know if they still lift the bridge or just leave it lifted. They took out the center section to have it refurbed in 2014-2017 so it can be done. I don't know if it is a month of work to do it or if just requires detaching the lift mechanisms and getting a couple tall cranes.
Although I think the designers are idiots to build that tall of ship with a known obstruction and not make the masts removable or able to be shortened.