Why many people is scared of branching? Why many development groups, even using a SCM system, decide to work on a single branch?

Problem with branching is that many of the RCS-descendant systems like CVS, Subversion or Perforce, and other very common like Source Safe, have big problems handling it. Creating a new branch in one of these systems implies replicating all the metadata on the new file, what is slow an inefficient. Besides it makes more difficult to know exactly what was done on a branch because everything is there.

Most of these version control systems deal with branches as if they were directories, so to start working with them you even have to organize the project in a special way.

Complicated.