Currently, if someone tries to access a folder they're not supposed to, they get the following text:

403: IP.Board -> Forbidden

You have reached this page in error, please use your back button to return to the forum.

IP.Board

But it only ever links to the IPB site. Why not change this so:

A: The page looks like it's actually part of the person's site, like any normal error page

and

B: It links back to the site home page?

Doesn't that seem to make more sense?

Pretty sure it's because it's just an html file.

The pages you are talking about are literally just index.html files (static files that cannot adapt per-site) that are intended to stop directory snooping on misconfigured servers. I, personally, would prefer not to include them (they make the download bigger and add needless files only because certain servers aren't configured correctly), however we do it as an extra protection for those who are not familiar with configuring their servers to make this a non-issue.