It's because we can't produce it fast enough. We can sell them 30 million rounds but if we only produce 50000 per week then they'll be dead by the time they get the 30 million.
Every week there are articles Ukraine is running out of ammunition, despite the huge sums of money they are getting. Hmmm...wonder why?
It's why we sent them the cluster munitions... We couldn't send them anything else at that time and had a bunch of Vietnam era cluster shells so that was what we had to send. We've been buying cold war era shells from 3rd world countries and dictators too. It's going to bite us in the ass.