Because they are like blind people who cannot see where they are going, they sin without knowing it. Being blind is a metaphor for a person being unable to see how wicked he is. (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here the psalmist describes people's thoughts as if they were people. He also describes the inner beings of wicked people as if they were a building in which those people could walk. Alternate translation: "in their inner beings they are always thinking about more evil things to do" (See: [[rc://en/ta/man/jit/figs-metaphor]])