That’s the crumple zone, the first out of five defence lines is kilometers away from that. At the rate of depletion, the offensive will burn out before it even gets to the first fortified line.
Meanwhile, US is only able to produce 20k artillery shells a month, while Russia is producing around 200k right now. It will literally take years for US to ramp up industrial production. Pretty much everything in the latest package to Ukraine is on order, it’s not coming out of existing stocks anymore.