Why does it take such a long time to build a new power grid transformer?
A geomagnetic storm of the magnitude seen during the carrington event would lead to massive induced currents in long distribution networks. The longer the network, the more induced current. As such the networks that are most at risk are the large international  networks, typically several hundred kV or more. These are custom built, at site, with literally truckloads of equipment.
While it is unlikely that it will be completely destroyed it will almost certainly be significantly damaged, and you can not simply ship a new one based on off-the-shelf equipment. It has to be build up from scratch.
Smaller stations will also be affected, but these are simple enough to rebuild. But: In a Carrington event there will be an immediate shortage of raw manterials and production capacity, that will be amplified by the fact that the locations building these parts could be without power themselves.
A powerful geomagnetic storm terrifies me. I have absolutely no faith in the current pro-forma testing and conceptual unverified protection mechanisms. Our modern society has never, ever, experienced a geomagnetic storm of those magnitudes. As seen during the current Corona crisis the experts can wave all the red flags they want, but in practice no proper action is taken.