Well, in California, it is. Brake fluid should not "take on water"
And yet, it does.
It's inherent to the nature of the fluid.
Your personal anecdotes don't trump basic chemistry.
Some light reading for you-
Brake fluid 101: What it is and how it works
Why do you need to change your brake fluid?
DOT 3, 4, and 5.1 brake fluids are hygroscopic. That means they absorb water from the humidity in the air around them. Even with the brake reservoir lid tightly sealed, eventually enough moisture will contaminate the fluid.
What's so bad about water in the brake fluid? Well, brake fluid is incompressible under pressure, and since it has a high boiling point, the heat from your brakes isn't enough to cause it to boil. But water has a much lower boiling point. If the moisture-contaminated brake fluid starts boiling, it creates gas bubbles. Gas is compressible. So when you step on the brake pedal and create hydraulic pressure in the brake system, instead of that force being transferred to your brake pads in order to grip your rotors (or drums) and slow your car down, that force is wasted compressing that gas.
In more practical terms, it could mean your brake pedal just sinks to the floor without stopping the car. That, friends, is very bad.
Water in your brake system can also cause rust, which can gum up the small passages in the brake lines or brake hardware, and cause your brakes to work improperly or even drag – a situation where the brake pads don't disengage from the rotor or drum, creating friction and heat, and perhaps causing even more damage.