Why can't scientists control the weather?

Sometimes it can be artificially rained when necessary. Clouds are a large number of small water droplets suspended in the air. When small water droplets in the cloud combine to form larger water droplets, raindrops will form and fall from the sky. Scientists can spray a chemical in the cloud, which will combine small water droplets in the cloud to form larger water droplets, and finally fall from the sky. However, it is not only difficult but also expensive to use this method in a wide area.