They are increasing production as fast as they can (which is not fast at all, it's more like slowly steering a huge ship towards the correct direction) because current prices are too high even when accounting for the historical oligopoly dynamics. They can easily increase their collective profits by making more.
RAM manufacturers don't increase production as fast as possible, because they've been through enough boom and bust.
Rapid increase in capacity leads to oversupply which leads to negative margins. They've been there before, and they don't want to go there again.
RAM manufacturers do routinely setup new fabs and decommision old fabs. Maybe they're trying to hurry up new fab construction in times like these, and they would likely defer shutting down old fabs or restart them where possible. But they're less likely to build new fabs that weren't already part of their long term plans.
They've actually not seen such prices before. DRAM now costs as much per Gb as it did around 2006-2007 - despite around 20 years of real technical progress since then! That's genuinely unprecedented.
As far as I know, they are merely shifting capacities from the customer market towards the data center market with minimal retooling. I am unaware of any of the three actively investing in new capacity. Some modest increase are planned but nowhere near what you would expect given current demand.
