I never learned much about evolution, so please do explain it like I’m 5
In: Biology
Some dinosaurs survived (birds) and they did start becoming large again until they were outcompeted by mammals who were now evolving on equal footing, and better able to adapt to the new climate of Earth
To that point mammals did start evolving into megafauna, but a certain species playing with new rules (humans and tools) paired with rapidly changing climate again made being huge not a good trait to have.
Another factor is time. It took dinosaurs almost 200 million years to reach their peak of size (Jurassic and Cretaceous periods), whereas current animals have only had ~66 million years. They may need another 80 million for the environment to be suitable for huge animals again.
TL;DR – being huge requires a lot of specific conditions that all happen randomly to occur at the same time.
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