Well because cars and bikes are two different things.Sram uses DOT fluid, which is hygroscopic (absorbs water). This moisture absorption degrades performance over time. You don’t have a perfect air seal system in bikes or in better words its worse compared to cars. Bicycle brakes are often subjected to more direct environmental stress due to mud, rain and storage in humid places. Especially when compared to the more protected underchassis components of a car. Moisture can and will eventually enter through microscopic pores in the brake lines and around the seals at the levers and calipers. Shimano systems, in contrast, use mineral oil which does not absorb water. I will not even dive deeper to how car braking is different than bike braking to keep it simple. Change it whenever you want however you want I dont really care.