Flushing the brake hydraulic system periodically is necessary because:

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Brake fluid is hygroscopic, meaning it readily absorbs moisture from the air and from within the brake system as it ages. That moisture accumulation lowers the fluid’s boiling point, so under heavy braking the fluid can boil and cause vapor in the lines, leading to a soft or spongy brake pedal and reduced braking effectiveness. Moisture in the fluid also promotes corrosion and wear of metal components and can deteriorate seals. Flushing the system periodically replaces older, moisture-laden fluid with fresh fluid that has a higher boiling point and better protective qualities, helping maintain braking performance and longevity.

Evaporation isn’t the main issue here, and flushing isn’t primarily about countering fluid loss due to evaporation. While brake fluid is non-conductive when fresh, its conductivity increases as water is absorbed, which can contribute to corrosion and degraded performance; that’s why removing moisture through flushing is important rather than relying on the property of the fluid when new.