In the past the dangerous morons sent their kids to die in war, and the people who stayed behind were the ones who were more peaceful
Historically, the number of people who have been offered a choice about whether or not their children would participate in a war is vanishingly small compared to those who have not.
Maybe it isn’t discussed because ARs are also the most common rifle in the U.S., and for at least 10 years now, the cheapest non-22LR. It’s hard to know how much of a role the psychological factors actually play when “easy to obtain” is a significant one of them.
“Easy to obtain” is also the part that is easy for legislation to address, while vaguely defined and hard to measure “psychological effects” requires significant effort just to understand, let alone implement the required social safety nets and induce cultural change to address the root causes.