To me its about heat, we know it gets hotter as you go down, we know volcanos spew molten rock.
Only where such massive heats meet the cold of space could a crust form.
A hollow centre surrounded by molten magma would be uninhabitable imo, unless of course it was venting the heat somewhere, which we would have noticed by now.
I dont discount there could be massive underground cave systems, but a hollow earth ?
Cant see it myself
Although the way things are going on the surface ATM, we may well find ourself retreating into Subterranean spaces, Human history...... from caves to ......caves...... how ironic
Having said that i think its a good scenario for preparation for space flight, any species that wants to live long term in space can train for it, by going underground, making their own air, food environmental sweet spot etc, and letting go the free ride, surface dwelling provides.
Its an artificial biosphere instead of a natural one.