STORY: Second chance

I stepped into the room, nervous but full of interest. "Don´t be scared, I won't hurt you, " she whispered quietly and moved a bit closer to me. Her wheelchair made an awful scratchy noise . "Hi granny," I said, waiting for any answer, but nothing has come. " Well... yeah, dad told me that, umm, you wanted to see me, so here I'm." She moved a little closer again. "Sit down, my little boy, just sit down and focus on the story I will tell you." Her voice was shaking and that made me even more worried.
"Long time ago, even when your parents weren't born, there was a big accident on the planet. But the planet wasn't the ours one - Mars. Its name was Earth. Our population lived there for a million years and we didn't do well. The problem was we all knew it, but no one really cared. We all were hoping for a brighter future, but nothing more. We were dealing with tons of plastics. Have you ever heard of plastic, my boy? No? Okay, let me explain it to you.
In the age of plastics, almost every single thing was made of this mysterious material, starting with bottles and ending with toothbrushes. People used to use it only once and throw it into the trash bin. After some time, there was no more space to place the trash on the land, so they decided to simply put everything into the oceans. And this was the beginning of the destruction. Of course, there were also many other problems like fires in forests, decreasing of animals or CO2 pollution, but the population on Earth used to speak about plastic, because it was the main thing they could help with on their own. In 2019 a little smart girl from Sweden, Greta, came up to the leaders and started fighting for nature and the future, but no one listened to her because she was only 16. She wanted to save the world, the humanity heard her but decided to not listen. And so are we here, Jeremy Thunberg younger. Your great-great-grandmother was a really strong woman, wasn't she? Don't you understand it? Greta was my granny, you silly. She took a couple hundred of us and got us to Mars about 100 years ago, because planet Earth now is just a dirty place full of dead bodies. But we survived! She has chosen us and given us the possibility to live. Aren't you happy?"
I felt like now I don't understand anything. So there were milliards of people on the Earth? Why were they so silly and didn't do anything?! Were they all blind?! So many questions and no answers, but I know one thing exactly. If I could travel back in time I would help Greta save the planet. So why didn't they?