Why would Seth have a Ghostbusters lunch box if he was in grade school back in the late 90's?

Ghostbusters II was released in 1989. Additional material was released at the same time. Ghostbusters had a pretty large fan following for years. It doesn't strike me at all odd that he would have items in the late 90's from Ghostbusters. I recall kids in the eighties and nineties also had original Star Trek and Star Wars boxes and other items through the nineties as well.

Nostalgia nods, pop culture references by the film makers. Why does Tony Stark wear a Bruce Lee T-shirt in Avengers? Why is there a Hills Have Eyes poster in the basement of the cabin in the woods in Evil Dead.

Someone behind the camera thought it was cool to add it in the movie.

Or from the characters perspective I can see him catching Ghostbusters on TV as a kid and being into early Bill Murray, Dan Ackroyd, Chevy Chase etc

Things had so much staying power back then. There was the Hi-C Ecto Cooler which every kid loved. Inspired by Ghostbusters with Slimer on the juice box. Every kid would use the line "Who you gonna call? Ghostbusters!" Ghostbusters was pretty much always cool.

This movie tries very hard to be dated older than it is. Clothes, soundtrack,cars,etc... They feel out of place.

Because Seth Rogen and Evan Goldberg wrote it but set it later in time.