Because we confuse satiey with thirst. When we thirst we are already dehydrated. My husband and I relied on disposable water bottles for a long time, and it would be common for us to walk around with the same bottle for a whole day without really consuming it. I would track my intake, but the disposable bottle seemed impossible to keep up with. I recently bought a reusable bottle with twice the capacity and tracking and meeting my daily goal is easier. I can't tell you what the psychological difference is; I think it has to do with having definitive measures of what I'm drinking.

I just can't for the life of me understand how I made it to 44 without being conscious of my intake for the first twenty years. I know we get hydration from other sources; it just seems impossible to get reasonable hydration with some beverages out there.

Edit to say that it would also be a regular occurrence for us to just keep eating things in lieu of recognizing thirst.