Because your body is a "natural"
water filter. The human body collects, processes, and filters
contaminants on a daily basis from the food, water and air that
we consume. If you drink water with high levels of pollutants,
your body then becomes a living filter of sorts. It will remove
many contaminants on its own but whatever it cannot remove will
be left behind to be absorbed or deposited in your tissues and
organs. Over time, the accumulation of toxic contaminants could
overwhelm your body's natural defenses leading to serious disease,
cancer and death.
It is easy to replace a bad cartridge with a new one when a water filter fails. It is much harder to replace any valuable organs when the human body fails.
Don't use your body as the filter!