They've been burned before. The DRAM industry has a long history of booms and busts.

Demand increased, everyone built new fabs, then prices dropped and they couldn't pay off their investments. Many went out of business. It happened in the 80s, it happened in the 90s, it happened in the 2000s.

Now there's only three manufacturers left, and they know very well that demand for their product tends to be cyclical.