Would you not benefit from the marketing even if the game was already released in this case for both skews.
In practice it doesn't usually work that way.
Imagine a major AAA game like that: "Coming to you in June for PC, August for XBox, and November for PS4."
Instead you get something like: "Preorder today to play on June 25."
If you market each one individually, you weaken your ad campaign. Now instead of one big ad blitz, you've got three major ad campaigns. And due to the way the hit-driven advertising model works, where the companies need to recover the costs within the first weeks or never at all, that means you want to have a single, large, comprehensive ad campaign. It is similar with the big budget movies where their opening weekend defines success or failure, not the potential years of licensing the movie out.
That isn't to say that alternate paths don't work. Many niche and independent games play the longer tail rather than the opening surge. But it doesn't look like that is what the original poster wrote about.