> Why is it when I add a page to my WordPress site, it doesn’t show up in my BuddyPress navigation?
Because that’s just the way the default home theme is written. The items listed on that menu aren’t ‘Pages’ in the WordPress sense, anyway, so it makes little sense for the default, out-the-box BuddyPress theme to do that.
The bit of code you’d need to change (or to add in your custom/duplicate theme) is in header.php. Where you see:
Add this underneath:
<?php wp_list_pages('title_li=<h2>Pages</h2>' ); ?>
Documentation for that function is at https://codex.wordpress.org/wp_list_pages