The ancient world exposed people to more stress and more events of stress compared to developed world inhabitants.
More stress => get used to stress => stress has less of an effect.
Just like any form of mental or physical training: Get challenged => Grow to meet challenge => The same challenges as before are now less challenging.
See: Stress inoculation
"The stress inoculation hypothesis presupposes that brief intermittent stress exposure early in life induces the development of subsequent stress resistance in human and nonhuman primates."
"People with the capacity to maintain healthy emotional functioning in the aftermath of stressful experiences are said to be resilient, or stress resistant (1, 2).
"Researchers have sought to identify attributes associated with stress resistant individuals, with the expectation that understanding the etiology of stress resistance may lead to the prevention of stress-related psychiatric disorders.
"One intriguing finding to emerge from this retrospective research has been that stress resistance is associated with childhood exposure to mildly stressful events (1, 3)."
https://www.pnas.org/content/103/8/3000