We constantly have spit in our mouth 100% of the time but if we were made to drink any amount of our spit it'd be disgusting. Why?

You are swallowing that saliva during the day. Usually we make 0.5 and 1.5 liters daily (for us measurements 2 to 4.2 cups). And that needs to go somewhere. And it is very important part of our oral hygiene too.