I would question the assumption here in several ways. For one, we just do not have the data to say for certain that PTSD was less severe from WWII vs. WWI. It's not as though modern therapists and psychiatrists are time traveling back to the 1910s or the 1940s to talk to veterans contemporaneously and collect statistics on diagnoses. The perception of PTSD rates in veterans from either wars comes entirely from impressions based on popular sentiment, which are likely to be very much inaccurate.

Additionally, it's important to recognize that during WWI they used the term "shell shock" as an umbrella term to describe a soldier who has been effectively incapacitated in terms of war fighting despite a lack of obvious physical injury. Since WWI we came around to the idea that "shell shock" was probably mostly a mental condition, specifically PTSD. However, in more recent years we've come back around and learned about the existence of traumatic brain injuries, specifically blast injuries. We know now that "shell shock" is a real physical condition and a non-fatal wound that results from being near an explosion.

WWI soldiers were subject to a lot of close artillery attacks, so it's very likely that the "shell shocked" soldiers of that war were a combination of those suffering traumatic brain injuries plus those suffering PTSD.

However, as I said, the data on PTSD for both WWI and WWII veterans is extremely spotty and nowhere near sufficient to be able to properly compare the PTSD rates and severity for veterans of one war vs. the other.