I found a bug in the Steam version of the game.
The option to choose local coop is missing, but I checked the same game on other platforms and the option is there. I think it might be a UI bug that needs to be patched so PC players can see the splitscreen option when they start the game.
For me, the bug reveals itself immediately on the main menu. The option to select local multiplayer (aka splitscreen) is not visible, while it is present on at least 2 other platform versions.
From the forums here, on Steam and Reddit, it appears to be affecting a very large number of players. Let me know if you need more help recreating this issue for your technical team.