Peter Venkman is such a presence in the first two Ghostbusters that suddenly diminishing his role would be a mistake.
It would admittedly be hard to build solid scenes around them, so Ghostbusters 3 needs a way to incorporate them while still having Venkman included to keep audiences entertained.
Especially given the franchise didn’t dedicate a lot of character development to Ray or Winston; they almost feel like dudes who are simply around to hold a proton pack.