I've worked only tangentially in the games industry, but I worked for many years in TV development. And people often asked why we (NBC or FOX, in my case) wouldn't accept unsolicited submissions of show ideas. The primary reason we didn't is that it was a legal minefield. If you read someone's submission, and you rejected it, and years later you developed a show with even the slightest hint of an element from that concept, the person who submitted it could sue you. And, even if he wouldn't prevail 9 times out of 10, he'd still eat up a lot of time and work and money in the process.
The problem is that there's really nothing new under the sun. Every basic plot imaginable has been conceived. Every core concept shy of the truly insane or inane has already been pitched (and even those have been pitched, trust me). The chances that your unsolicited submission will bear more than passing similarities to a concept already pitched, or already in development, are quite high.
Aggravating this problem is the fact that people have sued over the most facepalmingly generic bits of supposed intellectual property: character names(!), broad-stroke concepts (i.e., "I sent NBC an idea about superheroes; NBC made a show about superheroes!!!11!1"), and so forth.
At the end of the day, opening an unsolicited envelope is more hassle than it's worth. I can't say if this is a motivating factor for game developers or publishers, but I imagine it would be.