Because your building the most ridiculous strawmen to represent a domestic insurgency, and just asserting the stupidest fucking positions. You really think an insurgent is gonna walk into bass pro shop to restock ammo? Because there's crazy fuckers out there already stockpiling ridiculous sums of ammo. The US produces about eight billion rounds a year, and something near two billion of that goes to the US Military and its stockpiles. Someone's buying the rest, and its not lefties in soy shops. Insurgencies aren't going through ammunition at US military rates, if they are they've fucked up heavily and will be dead in the next fifteen and don't care anymore.
The more banal parts of insurgency supply lines come from sympathizers. Its not illegal for me to walk into a store and buy a loaf of bread. Its not illegal for my friend to give me a loaf of bread. I'm not gonna walk into the supermarket with my insurgency uniform on with "I Bomb Biden`s Balls" written on the back of my jacket. And the level of control that would be required to restrict food so much as to starve out an insurgency dependent on sympathizers, would just lead to more aggressive resistance, people would wig the fuck out if you had to let your home be searched to get a tightly restricted bread allowance. Its a losing game.
We have nearly half a century of solid proof that pretty much every standing military out there is simply incapable of fighting an insurgency effectively. And that's the whole entire point of insurgency. They bleed their opponents morale, a death of a thousand cuts. If the opponent tries to squeeze them tighter, it just leads to more people fighting back.