I think people tend to underestimate what's required to evacuate a planetary population. That's billions of people! The 24th century we see might have some massive passenger transport that could hold, say, 10000 passengers. You'd need hundreds of thousands of ships that size to evacuate the whole planet. And the logistics would be a nightmare's nightmare. A ship that size probably doesn't make planetfall. How do you get them loaded? Think about how long it takes to get a widebody jet boarded - maybe 500 people at most. What about the quadriplegics? What about the bedridden? What about the pets? Etc. etc. etc.