They're worried about people in the future who may not know about radioactive waste. This location will be dangerous for a very long time in a way that's invisible.
Carbon dioxide is certainly a concern, but most of its problems will never produce a place that people shouldn't stumble or explore because of unseeable danger.
I often think of the story of the Goiânia accident to be applicable. Four people ended up dead because two people stole something they thought might have value but was radioactive.[1]
It is prudent to assume that dangerous waste should be disposed of in a way that assumes there won't always be guards.