User content fields can only be customized within the additional tabs, configured in the field layout interface. Users are a built-in custom Element Type, and have some non-configurable fields.
This is likely a measure taken by the Craft authors to isolate and protect important fields from being lost among other custom fields.
It is possible to create front-end form that rearranges these fields, if the UX of the admin is unworkable for your users!
Edit (1/24/2018): In Craft 2.6.3005, the authors have added support for multiple tabs in the User edit screen. This doesn't change our ability to customize the first page, but allows you to add more tabs to organize the remainder of your fields. ✌️