I think the taboo of having contact with feces is pretty deeply hardwired into our brains, likely for the health reasons that Ms. Athena mentioned. So scat is an extremely edge fetish because that hardwiring is quite difficult to get past!
On the other hand, urine is a liquid and is sterile when it leaves the body (although pathogens grow in it pretty quickly if it just sits around- so don't put it in a mason jar and save it LOL!). Liquids have a different set of associations that include cleansing and purity... so urine being a waste product is kind of counterbalanced by those associations, at least to the point that golden shower play is much easier for many people to accept.
Just my .02, of course you're free to think I'm full of... well, nevermind!