it's not that they don't take it personally and actually feel the pain/ disgust / fear / <whatever>, it's that they hold the tenets of law as absolute and the most important part of the system. This definitely takes a particular type of person.
Many of the courtroom customs were partly influenced by this. For example the Robes/gowns or the emphasis on "law as an absolute", higher standard that sits above personal feelings is also inspired from Christian natural law ideas (i.e rulers and judges under a higher law) even though those ideas were built on Roman foundations.
So the kind of person who can treat the rules as more important than their own feelings isn't that surprising in that tradition. It requires a certain bit of.. religiosity.
whenever some unsavory person is in the news being defended by his attorney, most of us get upset, but they say stuff like, "his attorney is just doing a good job defending his client. that's how this works."
I get it, but I'm not built for it.
That'll fix everything.
If an obvious murderer isn't found guilty, I'm not upset at their lawyer. I'll be real pissed at the prosecution or whoever fucked up the evidence.