The equipment alone costs hundreds of millions of dollars. It doesn't make sense for manufacturers to operate these facilities at full capacity when not enough customers are buying their product. That's why major DRAM manufacturers may scale back production when there's a glut in the market and smaller players might shut down their Fabs altogether, or they'll switch to other memory products such as SRAM or flash. The result is less DRAM available for purchase. After a while, demand catches up with supply and memory prices go up.
Manufacturers then ramp up production to increase supply and the cycle comes full circle again.
Fab to full production capacity. And it can take just as long for the price of memory to react. The chips just keep getting smaller. This continuous reduction in size has dramatically increased manufacturers' ability to produce more product. Memory chips are made from silicon wafers.