I don't understand the 'hollow earth'

It looks like it's a (relatively) narrow zone with land below and quite closely above. How does that work? It's not really 'hollow', it's like a horizontal(?) cavity with equal gravitational forces above and below. It's confusing. And of course, where does the light come from?

They could have had a basic hollow earth with an internal sun and just one enormous spherical surface where gravity pulled outwards. The choice they went with is baffling.

The only reason for it seems to be for visual impressiveness and having Kong jumping between the 'top' and 'bottom' - which was kinda cool, I guess. One time.

Is there an explanation I'm missing?