The up-front investment of a memory fab is measured in billions, and takes years to construct and get running. The margin on the chips themselves is terrible, so without scale its not worth even trying. DDR5 is a industry standard that takes some effort to conform to, but the licence fees is a drop in the bucket to the cost of creating a fab.
The fabricators were cautious about increasing production, and slow to start planning. It takes further time to build up capacity, and if the demand drops down, they may end up producing dram at a loss when the market flips over to oversupply. The demand whiplash could kill any company that dared betting on increasing production. See the "bullwhip effect" https://en.wikipedia.org/wiki/Bullwhip_effect which has killed semiconductor fabricators before.
There is a discussion to be had about how to maintain national semiconductor production in Europe and US as a strategic industry, but historic attempts have all failed.
Now it's 2021 and someone gets a tanker stuck in the Suez, sending the price of oil sky-high. Do you spend those billions of dollars on a bet that it'll recoup before someone gets the ship unstuck?
If it was just variable costs and new capacity was available today they’d do it. But there are substantial fixed costs and delays to increasing capacity, and that uncertainty makes it risky.
The current RAM manufactures were convicted of conspiracy to manipulate prices back in the 2000s or thereabout, doing so is their modus operandi, but this time the government is participating in the racket.