EdgeCollider2D produces edges which have no area (they are infinitely thin lines) so don't affect rotational inertia at all, just like triggers. Have a look at the Rigidbody2D in the inspector, specifically the Inertia property and note it'll be set to 1. The rotational inertia of a trigger or collider with no area cannot affect rotational inertia so they add to up zero but because we don't allow zero we set to one.

So whilst 3 edges in the shape of a triangle might be visually the same as a polygon in the shape of a triangle it isn't the same to physics. A polygon has an area as it defines a closed area whereas 3 edges produce an open-shape even if you connect the start/end points therefore have no area.

This comes from the underlying 2D physics engine Box2D.

The user would have to set the rotational inertia in script in this case. https://docs.unity3d.com/ScriptReference/Rigidbody2D-inertia.html

Edges were originally designed in Box2D to be used in static set-ups i.e. they don't move.