For most people who say this, it’s just a rationalization.
Acknowledging that kids are infeasible for economic reasons, like GP laid out, is just too painful and embarrassing. So the ‘environment’ fig leaf saves their dignity.
It's kinda down on the list of prios but "I can not in good conscience have children grow up in this world where I see only problems and no solutions." is definitely on it, and you can call me overly pessimistic, but not sure how this has anything with dignity.