If mammals always have saliva in their mouths, then why do we get thirsty?

We get thirsty when we lose 1% of our bodies water supply. Not all water stays in the body for instance in urine, sweat and even blood if you were to get cut. Saliva is just one thing our body uses water in and is another form of water loss. As we cannot keep all the water in the body, although we are efficient as mammals with kidneys that try and keep as much water in the blood stream there is always going to be losses and therefore we become thirsty so we know to refill that supply.