Navigation is built separately from the page tree. Adding a page does not add it to a menu, and reordering pages does not reorder navigation.
Why the separation exists
It costs a manual step and buys a lot: menu labels can be shorter than page titles, menus can point at things that are not pages, and pages can deliberately sit outside navigation. The alternative — navigation generated from the tree — forces the site structure and the menu to be the same thing, which they rarely are.
The maintenance cost
Because menus are manual, they drift. When you unpublish or move a page, check the menus in the same sitting. A menu item pointing at a removed page is a link straight to a 404, and nobody reports those — they just leave.