In some areas , new builds etc have to have stormwater collection tanks .
Many of the new houses in my area have them , huge external tanks. Yet they werent allowed to be used for that homes water supply
Drinking water supplies have to meet much more stringent quality standards than generic rainwater storage, and would need to be isolated from the mains water supply to avoid backflow problems. It's just not feasible, at best you can use it for flushing the toilet and similar provided there's no way for it to flow back into the mains.