We all collaborate constantly, just think of all the help you get here when you run into an issue and all the scripts and models by other artists you use. Imagine what your game would look like if you had none of that.
Team requests however have caused issues in the past for the following reasons:
1. In-group fighting that is then held on the forums. Bad blood between community members.
2. Unclear/Neboulus rules about sharing copyrighted content.
3. Most notably: New users without experience trying to form teams to realize their ideas. While this sounds fairly innocuous it can cause a lot of distraction and wasted time. Sadly users pretending to have experiences in fields they don't and users pretending to run a studio which they don't is common with these types of threads.
4. Users recruiting people for a commercial project. This is a complicated thing that requires agreements, contracts and diligent observation of copyright laws (and laws in general, if you recruit someone underage for a commercial video game that can, in theory, be iffy from a legal stand point). These forums should not be a part of that.
We have nothing against people forming teams and maybe even start their own indie studio, not in the slightest, the forums are simply not the plattform to do so. If you are ready to form a team, you likely have IM client data or e-mail addresses of the people you wish to work with anyway.
-Wolf