Because like most thoughtless 24 year olds,all he probably thought about was getting home and whatever was left behind,his dad would see to it.
Do we know whether his dad DID put the gun away ?
Nonetheless, Lookout, which ever way we choose to see it, Jeremy was responsible for leaving out the gun he'd been using and it was used to kill 5 people. There are those amongst us who would wish, for ever more, that we'd put the gun away. Jeremy has said nothing.