The notion that collaboration makes people more creative has become conventional wisdom in the business world. Here's a typical example:

Collaboration has recently emerged as the defining characteristic of creativity and growth in nearly all sectors and industries. The singular genius who works alone is a myth of yesterday.

Despite this kind of corporate-speak truthiness, there is substantial scientific evidence that collaboration, rather than sparking creativity, results in group-think and mediocrity. What does result in creativity? Simple: solitude.

According to a study recently published in the Elselvier journal, the character traits of "shyness, avoidance, [and] unsociability," while generally seen as undesirable, are positively associated with creativity.

Furthermore, intelligent people are happier when they have less social interaction, even with their friends, according to a national survey of 15,000 respondents aged 18 to 28 and quoted in the Washington Post:

The more social interactions with close friends a person has, the greater their self-reported happiness. But there was one big exception. For more intelligent people, these correlations were diminished or even reversed. More intelligent individuals were actually less satisfied with life if they socialized with their friends more frequently. [Emphasis mine]

In other words, far from being a "myth of yesterday," the "singular genius who works alone" is much more likely to be creative than the person who seeks interaction and "collaboration." Forcing creative people to "collaborate" simply blunts their creativity.

According to an article in the Journal for the Theory of Social Behavior, there are

two ways in which solitude can facilitate creativity--first, by stimulating imaginative involvement in multiple realities and, second, by 'trying on' alternative identities, leading, perhaps, to self-transformation. ... By separating us from our usual social and physical environments, solitude can remove those people and objects that define and confirm our identities. The people we see and the places we frequent reinforce our identities as students, parents, police officers, or whomever. ... By extracting us from our customary social and physical contexts (or at least altering our experience of them), solitude facilitates self-examination, reconceptualization of the self, and coming to terms with change.

Put another way, being around other people keeps creative people from thinking new thoughts. Indeed, there are few experiences more mind-numbing for a creative person than being forced to interact with dullards on a daily basis. 

Even if your office is full of geniuses, they'll be less creative en masse than if they can work and think alone. In short, it's difficult and maybe even impossible to "think out of the box" when you're literally inside a box (i.e., an open-plan office) that's full of other people.