Why did the Spartans have a cruel upbringing?

Because the ancient Spartans had strict military traditions, in fact, Sparta itself was a military state. Thus, even in infancy, children were checked, if they turned out to be weak, then they were thrown off a cliff. At the age of 7, young men were sent to study in military camps, where they learned to survive. After that, at the age of 13, they began to undergo military training, at the end of which they became real warriors. Naturally, not everyone went through such a difficult path.