Because the earth isn't designed to take care of us. If we had stayed a small pre-industrial society, we would still eventually die of something - there are myr

Because the earth isn't designed to take care of us. If we had stayed a small pre-industrial society, we would still eventually die of something - there are myriad ways species go extinct. The only way to ensure long-term survival is to develop science and technology quickly enough to solve problems (both natural and artificial) as they come up. This includes solving problems created by past generations, who necessarily did not have access to the knowledge needed to avoid creating those problems in the first place. It's certainly not guaranteed that we will do so fast enough.