Underground isn't an infinite space. Power, Sewers, basements, vaults, foundations, and existing tunnels.

In London, building a new tunnel for crossrail involved threading a TBM through a gap with 80cm clearance (that's less than 2 cubits).

The deeper you go, the wider the problems above, the more existing infrastrucutre is effected, not to mention the issues of how to return to the surface.

Build decking over all the existing surface streets, only allow electric vehicles on the (current) underground streets, don't allow anything larger than a pushbike on the top.