There are two important factors you are not considering. The first is that it takes a long time train soldiers on complex weapon systems and the entire logisti

There are two important factors you are not considering. The first is that it takes a long time train soldiers on complex weapon systems and the entire logistical supply chain required to support and maintain those weapon systems. Giving someone a weapon system without this makes the weapon system nigh useless to them. Look at how long it takes for American soldiers to become rated on new weapon systems. The second is that weapon systems are designed to fit a specific war doctrine. Most weapon systems will have greatly degraded effectiveness deployed outside of the context in which they were intended to be used. US builds weapons for the way the US expects to fight. One of the reasons there is a shortage of artillery and related munitions, which is critical in Ukrainian doctrine, is that the US and many other NATO countries don't need much artillery and therefore don't have much of it nor the ability to manufacture vast quantities. And it is not feasible for Ukraine to radically change their war doctrine at this point such that NATO weapons would be more fit for purpose, it requires years of preparation to do this. Basically, the US but more broadly NATO can't easily supply a war that is being fought the way the Ukrainians are fighting it because their militaries are not equipped to fight a war that way.