I'd argue that pre-industrial warfare was even more brutal than today because warriors literally hacked each other to pieces. But I've never read anything about these guys suffering PTSD.
While PTSD probably was a factor, there are several factors that would reduce symptoms in the ancient world.
First of all, literally hacking each other to pieces is an exhaggeration. During warfare of this era, most died once the lines had been broken and on side started running - killing happened from the back in pursuit, not through hacking each other to pieces while in an organised line. Cavalry did most of the killing during a pursuit, and cavalrymen would strike enemy solders from above and behind and often not see the effects of their strike as the enemy soldier would fall over and they would continue onwards at relatively high speed.
Battle was a rare occassion during ancient times. For the winters, armies were either demobilised or camped in winter camps, and a war with a battle every year was considered an intensive one. Alexander's invasion of Persia had three great battles - the Battle of the Granicus 334 BCE, the Battle of Issus 333 BCE and the Battle of Gaugamela 331 BCE. Three battles in four years.
Battles usually lasted a day or two, including pursuit. This meant that while they were extremely stressful, troops had plenty of time to recover from this stress - up to two years - while working at far less intense endevours, such as foraging, building siege equipment, training new recruits, making and breaking camp, marching, tending to their equipment and so on.
Today warfare is much more intense, and combat keep up for a very long time, creating a maybe less stressful environment in the short run, but keeping it up for far, far longer. There is much less down time in warfare today than there was back then.
Grossman argues in his book "On killing" that slow demobilisation, or allowing soldiers to work through their experiences together with other soldiers before returning to civilian society reduces PTSD symptoms - it would take an ancient army weeks, maybe even months to get back to their homes and in many systems, they would keep in their unit and their equipment until they actually were home.
It was not until the Vietnam war that a veteran could be out of his unit, back in civilian clothing and then flown back home in less than a few days.
I'd argue that due to the much slower rate of movement of ancient armies, the much lower pace of combat (with a battle once a year or every two years) soldiers had much more time to deal with the extreme stress of combat before returning home.
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