system
Given the following post, respond as if you're commenting on Reddit:
human
[Attack on Titan] Why don't people live underground?

They have gun powder or black powder technology, why haven't humans adapted to live underground in bedrock or mountains, rather than in extremely vulnerable and concentrated dwellings?

They could have huddled up in a cave and expanded it over time.

Or even occupying old abandoned mines?

It seems like a narrow cave opening would provide infinitely more protection and require a LOT less maintenance than a 300 foot wall in an enormous open field?

Plus it seems like they would be able to incorporate offensive hit and run tactics more effectively through strategic placement of tunnels (i.e. In wooded areas).
gpt
They need to farm. It doesn't benifit you any thing to be in a secure place with if you have no food and will starve to death shortly. 
Given the population size they must require quite a lot of farm land which would require walls to protect. Basically they require the current system for food so living in a cave doesn't increase your chance of survival just your chance of a slow lingering death of starvation.