It's ultimately not a matter of how easy, it's about the limit in materials. Sure, you can find stuff in a city but there with likely be thousands of others searching for the exact same stuff, and as previously mentioned those supplies and raw materials will not be replenished due to likely economic collapse. It WILL run out, and I doubt your group will have the mining tools need to get more from a mine (as that will be the only reliable source of it
As for why to avoid cities, it's due to the principle of "the best way to survive a fight is to avoid one". Cities are likely to have the largest concentrations of the undead, so every time you walk out the door of where ever your safehouse is, you're taking a massive risk of dying whilst out, or even drawing the undead to your safehouse. That's the second major problem: Entrapment. You could run into the situation where the hordes have found out your location and trapped you inside, and unless your group already has a weapons stockpile capable of starting a decent sized ground war, you are going to starve and die. By being out of the city, you reduce the chances of running into zombies, as well as the odds being the groups are smaller so you can deal with them piece meal as well as give yourself more room to maneuver (or escape).
Also you need to take into account morale, as that's the real killer here. For every person you lose (and with taking that many risks inside a city, you will lose people) the morale of the group will drop like a stone, decreasing their combat effectiveness and making it more likely that you will lose more. It's a vicious cycle, and one that no group can escape when they are in the Apocalypse.
I'm just going to make a bit of an assumption here from what I've read of yours and ElPatron's posts. From what I've gathered from our posts, you guys seem to think that most of what you try is going to succeed, and that's a fatal mistake. Any military individual will tell you that no plan survives contact with the enemy, and in a situation where there are so many unknowable factors (such as the zombie apocalypse), you'll be lucky if half of what you plan goes right, and that's an inexcusable percentage when you have so little resources and manpower to begin with. For example, what if your gunsmith friend dies in the initial infection? Suddenly your entire plan of maintaining a weapons stockpile crashes around your ears, and I doubt that your ideal group knows off-hand (that is without checking youtube or any other resource for information, as those will swiftly become inpoerative) how to make and maintain guns and their ammo.
I however assume that things will go wrong at the worst possible moments, so I cover for that by trying to avoid as many confrontations with the undead as possible. That's why I endorse moving out of the city, that is why I endorse trying to use materials from renewable sources for weapons and traps that are so easy to make a child out unaided could construct them. I'm trying to cut down my odds of fighting and resulting failure every chance I get, which raises my odds of survival, even if only by a minor margin. And it is those minor margins that can mean the difference between life and death and undeath.
Oh and I'm going to leave you with a question: If headshots are so easy, why is it that in most combat situations they are so rare? Food for thought....