A "co-driver" from the destination country gets a full day salary just for that task.
The driver has to be shipped from their depot by road to the station on the opposite side of the border, take the train on both ways, then return back to post (again by road).
By the time of arrival at their depot, they either have no trains to drive, or are already way into their shift to take care of one, so they are dismissed.
This doesn't happen with the Celta Porto-Vigo, since both CP and Renfe drivers are already qualified to run on both networks, as the journey goes well beyond the station right next to both sides of the border.
Medway and Takargo drivers also have such qualifications to operate cross-border.
But it's true that "to the border" services between Spain and Portugal are basically going extinct. Renfe used to have Regionales that last called at Valença, in Portugal, and the Sud/Lusitania Express required a Spanish diesel locomotive to travel to the Portuguese side of the border and swap with, or from, a Portuguese electric locomotive, because the Spanish side is neither electrified, nor do they use the same electrification as Portugal (3kV DC vs 25kV AC).