You'll note that there was one completed in the 2010s and one in the 2000s. There were 33 completed in the 1920s.

The US produces 30 times as much as it did in 1920, in real terms. It is "economically plausible" for a nation 30 times richer than it used to be to produce some tiny fraction of the infrastructure it used to produce.

Californians might not want to build reservoirs, Californians might be too incompetent to build reservoirs, Californians might prefer blaming a snowpack that melts out slightly earlier to building reservoirs, California might prefer saving snails and slugs to building reservoirs, but California, undeniably, has the money to build reservoirs.