why do dealers tell me that every two to four years in a new car, that the brake fluid has to be changed because water gets in the system?

Brake fluid absorbs moisture from the air which can damage the braking system. This service is a good idea. 4 years is good in a dry climate but do it more often if it is humid where you live.

Unless you plan on keeping your car for a real long time, like 10 years, or 150,000 miles , this is a complete waste of money. Yes brake fluid as designed is hygroscopic, meaning it absorbs moisture, the dealer charges you for this service because it is easy to do, and costs almost nothing to perform. I only bleed brakes ( change fluid) while i am making a brake repair. It does not need to be performed every 2 to 4 years.