One of the reasons is we would need to know all their DNA, and the earliest DNA we have found is from a million years ago. However, dinosaurs lived over 66 million years ago
As Samuel said, the big problem is that we don’t have all their DNA – so it would be like trying to bake a cake with no recipe or ingredients. There is also the problem that the world was very different when they were alive, so they probably would not be very healthy or happy even if we could magically make one now
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