Probably the cost of a gadget that detects the extra d.c. and injects and equal and opposite current to null it out, for every major power transformer in the land.
Or just breaks the circuit, like a fuse. Better for the grid to fail in a way that can be recovered from quickly than for it to fail and burn itself to the point that recovery takes years.
Apparently more cautious operators have been busy installing them since at least 1989.
https://assets.lloyds.com/assets/pdf-solar-storm-risk-to-the...