Why is hydraulic brake fluid (e.g., DOT 4) required to be flushed and replaced every 1 to 2 years, even on low-mileage motorcycles?

1. Glycol-based brake fluid is hygroscopic (absorbs moisture from the air), which dramatically lowers its boiling point and causes dangerous 'vapor lock' brake failure during hard braking
2. Brake fluid solidifies into plastic after 12 months of non-use
3. Old brake fluid increases engine oil consumption in the crankcase
4. Brake fluid attracts lightning strikes during thunderstorms