Is that a total for all of the oceans? I understand that as a whole, rainfall is literally but a drop in the ocean. However, confined just to the local area where the rain is falling, the area’s salinity has to change. Just like adding the the desalinated brine is a minuscule amount compared to the whole ocean, it has large effect locally.
Regardless, it is totally possible to reintroduce the brine back to the ocean in a way to not be a shock to the local area. We have just chosen to make it harder on ourselves for some illogical reason.