Well tbf, they don't totally scratch them off either, it just isn't the main focus of the show. Some of them have been addressed throughout the seasons,

• Joyce in season 3 unable to move on from Bob and getting his death flashbacks upon reentering the HNL
• Hopper and his Sara flashbacks and how he spent most of his days drinking until Eleven entered his life. (He also mentioned almost shooting someone's dog because it rushed him and he thought that it was a Demodog, so that's PTSD from the season 2 events)
• Max with Billy this season and all the trauma and PTSD she has
• Will and his episodes in season 2
• Nancy blaming herself for Barb's death and having a breakdown in her parents' bathroom

If we are talking about the kids watching the agents die in front of them at the school near the end of season 1, then yeah that would realistically have a bigger effect on them, and maybe it did between season 1 and 2. Again, it just isn't the main focus, but they address trauma good enough.