Collaborative learning- some questions

This is a short post.

Why when we talk about collaborative learning do we usually refer to online activities rather than face-to-face small group work?
Why do students see the value of discussing a topic and learning from each other face to face much more than contributing to a wiki?
Why does everyone, including educators, find online collaboration hard?
Does the interaction that wikis produce actually facilitate learning?
Why do we worry so much about assessing online collaboration when we are happy for students to work in small groups in a tutorial without assessing relative contributions?
Doesn't the focus on assessment rubrics mean that we will make the students focus on external motivators for particpation rather than internal?

I have many more questions but it feels good to get these of my chest for now.