Water is pretty heavy (eg, denser than wood, denser than oil, denser than fat, etc).
Weight is costly (eg, mv^2 energy to accelerate). I guess human bodies have spent a lot of time trying to find the minimum amount of water necessary rather than trying to build up a reserve.
The little micro-optimisations add up over time. The penalty for excess weight doesn't have to be large before evolution tries to get rid of it. Historically, fat people are an anomaly.
Weight is costly (eg, mv^2 energy to accelerate). I guess human bodies have spent a lot of time trying to find the minimum amount of water necessary rather than trying to build up a reserve.
The little micro-optimisations add up over time. The penalty for excess weight doesn't have to be large before evolution tries to get rid of it. Historically, fat people are an anomaly.