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?

It's the quantity. And also, by the time you've filled a cup of spit, it has cooled and dried a bit, maybe even breaking itself down, resulting in an increased viscousity, giving it a very unpleasant mouthfeel.

I feel the same way about cum that isn't directly from the "tap." Somehow it's fine from the source, but you put it in a shot glass and suddenly it's disgusting.

Same reason you wear your socks the whole day but once you remove them they become eww. Human brain is weird in that regard. Probably has evolutionary reasons but from an entirely rational POV it's weird.

I had some oral mucositis that caused some excessive saliva production. There was one time I accidentally swallowed a mouthful of it, it immediately caused me to vomit.

Thanks...now I'm acutely aware of the spit in my mouth and when I swallow it.

Fluids that exit the body is like this. Only exception is blood from your own kin and tears. Anything else gets very dirty once out of body. Sweat, urine, vomit etc

Spit in a cup and drink it and watch how your peers react.

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.