Why don’t animals roam to new territories one by one, and only as part of a flock or herd?

Animals do not roam alone, as they themselves cannot survive. The main task of any living organism is to survive and produce offspring. If the animal survives, then on the new earth it is impossible to get offspring alone. In addition, when roaming in a flock, there is a high chance of counteracting predators or distracting predators with the help of weaker animals. This happens very often, especially in Africa when zebras or other animals cross a river inhabited by crocodiles. So all of this has an important biological meaning.