> why can't we just by default build the reactors underground?

It would appear to carry an increased risk of hidden problems and groundwater poisoning. There might also be issues with corrosion and accessibility for maintenance.

That said, I'm not ruling it out. A lesson these new approaches to nuclear engineering is that we never figured out the right defaults the last time around. We were too focussed on making bombs and submarines.

> we already know the impacts of underground nuclear explosions

Nuclear plants should never fail by way of nuclear explosion. If this is a possibility it is a bad reactor design.