Even FEMA doesn't recommend you store that much water - they were recommending one gallon per person per day for three days before considering anything else (climate, type of emergency, children/pets/elderly/sick people). Plus you have to make sure that it hasn't expired or gone bad... 40 gallons is a lot more than people will consider storing. Hell, I don't even have space for 40 gallons of water anywhere. I barely have any room for a 3 day stash of food/water and a first aid kit. Plus maybe some of these people do have a good amount of drinking water stored up for an emergency, but there is no ETA on cleanup yet so they have to prepare for the worst. Even 40 gallons isn't enough if this goes on for a week for a family.
Also as a co-owner of a small fast food restaurant (in northern california), I can't use stored water for anything related to running the business. 40 gallons is a literal drop in the bucket in one day of use too, let alone however many days it will take to clean up the spill. That's a lot of businesses closed right there because they literally can't do anything to prevent that from happening. That alone, even before considering individual water needs, is a big deal.