Because tools are not invested with the will to collaborate.
Because before tools can aid collaboration people have to wish to collaborate.
Because some people would not collaborate if anything less than the very success of their endeavor is dependent on it.
Because for every person with an ounce of ego, the perfect solution already exists in their head, if only reality would yield to it.
Because metrics that measure success of their objectives do not account for the pathways taken to accomplish them.
Because every fiefdom has its lord who is reluctant to mingle with the peasants.
Or we collaborate so there are co-conspirators who can stand along side when its all in shambles.
What other reasons do you think make large enterprises have sub-optimal levels of collaboration going on?