Why use shards?

[Marmalade]

So, let me make one thing clear: The only experience I've had diggin' in the guts of DST is modding in characters, structures, items ya'know, basic stuff. What's stopping Klei from exploring the y-axis a bit and just generating the entirety of caves right beneath the forest in game? I feel like it would definitely save loading times as transitioning from the surface to the caves would simply be loading in the caves to actually be accessible, teleporting the player far down below, and unloading the surface. I feel like I'm seriously missing something here, so enlighten me.

[Auth]

I believe it was originally done because it was difficult for them to make sure areas had the correct lighting during different times of day (Like how do you keep the caves dark during the day but the rest of the world not without obliterating performance), and maybe something about the nightmare cycle, but I may be misremembering. It's been a while. I think Klei could probably do something about it now, but that'd be a pretty intense overhaul for something that's still generally just kinda fine as-is. Would be awesome though.