Big Download contacted Atari about the absence of multiplayer support in the PC edition of Ghostbusters, and Atari passed along a statement from Dan Irish, CEO Threewave Software, who created the game's console multiplayer component. The obvious reason for this is that the console editions were a higher priority, but at least he comes out and admits that. Here's that bit:

Multiplayer for the PC version of Ghostbusters is something that Threewave and Terminal Reality always wanted to do. However, our focus was on making the console version the best that they could be - an experience that allows multiple players to re-live and re-fight many of the themes from the movies. When we looked at the resources necessary to pull off multiplayer on the consoles and the PC, it became a question of what could we do and still be excellent in our execution.