That is by design in Access.  You don't have to put all fields on a form from a table.

So...  If you add fields to a table and you want them on the form all you have to do is click the Add Existing Fields, while in design mode, and drag the new fields to the existing form.  No need to start over.

Dragging the Table to the Form creates a Subform. Re-watch Access Beginner 2 which covers proper Form Design.