WHY must a hexagon add up to 720° (in pictures)

Because there are now 4 triangles in a pentagon, it must add up to $4times180°=720°$ .

A hexagon can be divided into 4 triangles

The hexagon can be made from 4 triangles.

Example 1

An equilateral hexagon will have 6 equal internal angles and would be

$720^circ/6=120^circ$

Each internal angle is 120°.

Each angle of a hexagon make 120 degrees

But the interior angles would still add up to 720°.