Easy. I already said, you should try reading my post...but here goes again: Christian faith requires quite a bit of mental gymnastics, and leaps around logic and fact. Yet, art baffles you.
Before you jump to any conclusions, here's some context. I was raised in a strong protestant household, and much of my family are still church going protestants. I also graduated from a Christian school. So from 0-18 years old, I was saturated in Christianity. But what did I lack? Faith. I couldn't do enough mental gymnastics to ignore the questions that can't be answered and just believe.
Ok, I hope you made it this far in the post.
You should view art like Christians view the Bible. Art is art due to the perception and/or interpretation by the person viewing it. That's just what it is.
This can also be compared to Christianity. View Art the same way Christians view the Bible.
How do you think Christianity has been split into so many denominations? Perception and interpretation of scriptures create different view points by man.
So for you to sit there and say "I don't get why this is art" is either:
A. No one has ever explained to you what art is, will be, can be, etc...
B. You're being intentionally daft for the purpose of this Pissed Christ thing.
edit: googled "Art Definition" for you also:
https://www.google.com/search?q=art&ie=&oe=#q=art+definition
Google is your friend.