Short answer: because no one would bother create another account just to post comments anyway.

The kind of sign up forms we all love.

I did a little survey about features that are really needed in a CMS. To this end, I downloaded the full list of top-100K-Wordpress-powered-sites, and randomly visited 20 sites (ok, I should redo on a higher number of websites to have meaningful statistics).

The result is that 18 of them :

• didn't use comments at all (see this random site),
• or all their articles had 0 comments (another one),
• were written by one single user, usually the site's admin: most websites don't have a community of redactors (even for corporate websites, the work is often done by one single person)

That's why Void doesn't provide "login", "sign up", "comment" features out-of-the-box. You can always add it yourself (but it will become heavier and out of the 100 max lines of code philosophy), or easier : use Disqus, like we did here for this actual website.