There is no need for a Harpoon, the weapon would only be necessary in a war that involved conflict against a peer Navy, and even then, it is unlikely the US would fit launchers to any ship that doesn't have them.
Boeing can build several dozen Harpoons a week if required, and the newly built (and purchased) Harpoons would be of Block II (or soon to be Block III), which are VLS launched, not launched like the older Block Is currently deployed on USN warships.
In other words, as a weapon that can be mass produced within the United States quickly, the Harpoon is a weapon that the US Navy chooses to go without despite fully funding research for the weapon (which is exported to allies), but as the paying research customer reserves the right to buy and build in mass at anytime.