Because when I worked in the motor trade we invested in an expensive piece of kit to test brake fluid. After testing hundreds of vehicles with fluid as old as ten years and looking shitty, we found that not one of these vehicles had any issue with the fluid that required it to be changed.
Added to this, BMW change the fluid at the first year, whereas other manufacturers, using the same fluid, same ABS systems, same brake lines, same callipers don’t do it at year one, most are year two and some are year three or more.
Also most motor vehicles don’t change the fluid for many years, quite a few over 5 years, some 7 years and even the yanks make cars with the brake fluid sealed for the life of the vehicle.
That is how I know.
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My MK4 Mondeo was filled with long life brake fluid - good for 10 yrs apparently.