Northrop Grumman makes weapon systems, fighter jets and cruise missiles. Not stuff getting fired from a cannon. Shells are cheap and so are the margins. They are still using some designs from 75 years ago.

The US just doesn't fight wars with cannons(if they can help it, in sure they used them plenty when occupying Iraq and Afghanistan). The Ukrainians are though and it's using up a lot the available shells at a rate the US and NATO can't replace them at. They have increased production but nothing near the rate they could if they really wanted to.