How do you know you're thirsty? For a start, there are a lot of medical and neurological issues that can stop you feeling thirsty (diabetes etc). A large percentage of the population don't have the correct hormone response that tells them to drink. Secondly, especially in Western societies, there are social conventions that mean we always have a drink with food. That creates a conditioned response that means many people literally mistake thirst for hunger, so when they feel thirsty they eat something. (That also contributes to obesity.)

'Drink X glasses of water a day' is a simple adage that mitigates all these problems with very little risk.

For whatever reason that has just never worked all that well for me. I can be significantly dehydrated, and not really feel very thirsty.

I have the same problem with food though, I can be 3,000 calories under my daily BMR and not really feel all that hungry.