Jeez...really? The simple answer to this is : Because the owner of the Forum WANTS it that way.
This isn't YOUR forum. Nobody is duct taping you to your chair and forcing you to interact exclusively within this site. It is a forum for information sharing and communication among individuals with similar interests or at least a similar current interest. Like all organizations it requires rules and a system of regulation. You may not LIKE the system that has been put in place, but again, IT ISN'T YOUR SITE. It was built, registered, and hosted by someone else. It is THEIRS to do with as they see fit.
If you don't like it? Create your own forum, set up the rules to your liking, and don't bother with this one. That's the beauty of the internet. Otherwise simply be grateful that the site exists, adapt to the rules in place, and enjoy it as it is.
What is it with people thinking sites like these should be some sort of democratic republic, in which they should have a say in the operation, when they didn't do a dang thing in the creation of, purchase of, or promotion of the site??