Because they are not clearly identified as a soldier.

Specifically (III.Art4.2.b): that of having a fixed distinctive sign recognizable at a distance

Although:

(6) Inhabitants of a non-occupied territory, who on the approach of the enemy spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units, provided they carry arms openly and respect the laws and customs of war.

...might be a getout clause, provided that they then organise themselves.