Brake fluid is hygroscopic, which means that it absorbs water from the air. Estimates are that it absorbs 2-3% water per year. Water doesn't compress as well as brake fluid, and also causes corrosion in the system. If it was better or the same as brake fluid, trust me the manufacturers would be filling brake reservoirs with water to save a penny! It is also why you should never use the contents of a previously-opened bottle of brake fluid to top off your car.
In a normal car, the brakes generate heat with every stop. That heat helps to boil some of the water out of the brake fluid, since water has a lower boiling temperature than brake fluid. Manufacturers who specify fluid change intervals on ICE cars frequently use 3 years, regardless of mileage.
On an EV, regenerative braking limits the waste heat that boils the water out of the fluid when you're not using the brakes hard. The BMW-specified two year change interval regardless of mileage sounds reasonable to me!
Given that the two year brake fluid change is the only scheduled maintenance on the BEV model that I know of, it isn't exactly going to break the bank to pay for this service every two years. This and the scheduled two-year dealer inspection cost me a smidge over $300 last month when I paid out-of-pocket for my new-to-me 2015 i3's scheduled fluid change. I'm afraid to look up the cost of a replacement ABS pump, to see how much it would cost me if it was damaged due to corrosion from water in the fluid.....
2015 BMW i3 BEV, Giga World, Tech and Driving Assistant packages, 15K miles