1. There's a lot of great conversation in forums that requires login if you want to participate.
2. No login=no user session tracking and the protection against spam this helps bring to forum owners.
3. Many forums don't have feeds because they don't want their content duplicated on other sites.
4. Many sites don't allow searching their database unless you're logged in.
5. Many forums use registered user counts as a marketing tool and thus require login for full access.
6. Big players/authority forums don't want share universal logins. It helps small time player far more than big players.
7. Interaction: sure you can read many questions and answers of others but what happens when you need to ask a new question?
8. Some sites actively block crawlers that suck up bandwidth but bring few visitors and little ROI. Sure... you can use data collection tools to categorize content but not if the tool is blocked.
9. How would you be able to ask great questions like this thread if you didn't login to WebmasterWorld?
10. Interactivity doesn't work if the key component is missing: interaction.

I'll give you one good reason:

What you post on an active forum will get seen by many people who are interested in the topic. What gets posted to your blog most likely won't be.