Because this is how they feel about themselves deep down inside. They feel ugly and miserable, trapped in a pointless joyless world full of trivial nuisances and petty indignities.

They know that they had their time and they they fucked up. They could have left the world a happier and brighter place but instead they left a planet boiling to death just so they could build strip malls and dumbass McMansions. Even if they wanted to build a better world it would be to late for them now to see it now.