You do need to remember, the US is a long way from dipping into its existing stocks to the point it can't easily replace it. Yet.
To ramp up new or mothballed US production lines is still going to take another 12-18 months.
European NATO nations have certainly dipped past the point of production not meeting demand, as they had less to begin with. Which is why South Korea has stepped in, as it has both has huge stocks (not as big as US though) and the existing manufacturing production lines.
Also components to make one 155mm round (shell body, explosive, GPS guidance and powder charges) come from different countries, so worldwide shipping issues (ie lack of water in Panama Canal currently) is a killer.
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