WHY must an octagon add up to 1080° (in pictures)

Because there are now 6 triangles in an octagon, it must add up to $6times180°=1080°$

An octagon can be divided in 6 triangles

The octagon can be made from 6 triangles.

Example 1

An equilateral octagon will have 8 equal internal angles and would be

$1080^circ/8=135^circ$

Each internal angle is 135°.

The internal angles of an octagon equals 1080 degrees

But the interior angles would still add up to 1080°.