Because I can tell the difference. The best tasting water I’ve ever had was in Vail Colorado, and it was tap water. For months I thought the water in Vail tasted terrible, and I hated drinking it. One day I accidentally left an opened bottle of water in the refrigerator overnight. The next day it tasted amazing. I think the chlorine and/or other chemicals evaporated. It’s not easy to describe how water tastes in general. Maybe it isn’t even a flavor, maybe its minerals interacting with my biology. The taste comes at the end or finish of a sip, it was crisp and weirdly almost dry tasting.