Armed citizenry are a mob, given whatever guns are to hand, with brief training that the muzzle end is to be pointed towards the foe. Weapons do not make soldiers, and arming the people is an act of desperation. On the battlefield, the citizenry are unlikely to do much to damage an enemy, but the size of the mob can at least cause the foe problems. This can give real soldiers enough time to win the battle, or make the enemy pay heavily for their victory.
Historically, arming the people was something that really scared the authorities in every nation, particularly if the population hated the existing regime. It was not wise to supply an angry population with weapons, when these might be turned on the rulers, rather than foreigners. When Austria decided to create landwehr or militia forces, it was careful to exclude many regions from the decision as the loyalty of Austria’s many different ethnic groups could not always be guaranteed.