Multi layered topic.
On one level, it is because our cities and agriculture use and waste water as if it were an abundant and cheap resource. That's why all the shower water, dishwater, sink water you use - 75% of your household water use - gets discarded to the ocean instead of reused for toilets, irrigation, industrial uses. That's why crops are grown in the Central Valley with spray irrigation and no evaporation barriers. That's why people living in deserts have bluegrass grass lawns. If the US used water at a gal/ household rate like most other countries, there would be no shortage of water in California, or Texas for that matter.
Average Water Use Per Person Per Day
On another level, there would still be a drought, for the forests and mountains and rivers. That is partly cyclical and partly climate change-driven.