It's about capacity. When ammo was plentiful, peoplenwould only buy 1-2 boxes at a time. Now that it's hard to get, people buy all that can find! Same goes with primers. On top of this, we have millions of new shooters and tons of new reloaders.
Sure, they could build new plants to increase capacity, but what happened after Trump got elected? People stopped panicking, and STOPPED buying. They simply started shooting up what they already had and didn't buy anything. There was a supply glut and prices dropped to super cheap as there was more supply than demand.
If someone builds a new plant, they are taking a huge chance. They may either end up with a plant they don't need or the Democrats might find a way to throw a branch into their spokes right after they make their huge investment... screwing the entire business over.
This being said, the manufacturers are surely making record profits. They COULD build other plants. But if the dems are smart, they would encourage them to do so, then open the door to foreign ammo again. This could put them under, especially if we go Republican again and people stop panic buying.