There is good reason for that: the US/NATO war plans are not to get into an artillery war in the first place. If there is artillery in the way the US/NATO plan is send an airplane with a few bombs to take it out. There is still some room for artillery in the army and so we produce some, but that isn't the major way to fight wars.
The Soviet plan - which both Russia and Ukraine are well trained in - was to use lots of artillery. In backing Ukraine NATO suddenly sees a need for some shells that they wouldn't use if it was them. But the Ukrainian generals know them and so that is what they want. (Note too the nobody has provided Ukraine anywhere near the number of airplanes needed to fight a NATO style war - even if all promised F16s arrive today with full training it isn't enough for a NATO war)
The Soviet plan - which both Russia and Ukraine are well trained in - was to use lots of artillery. In backing Ukraine NATO suddenly sees a need for some shells that they wouldn't use if it was them. But the Ukrainian generals know them and so that is what they want. (Note too the nobody has provided Ukraine anywhere near the number of airplanes needed to fight a NATO style war - even if all promised F16s arrive today with full training it isn't enough for a NATO war)