They piss on it because Christianity is full of outdated expectations based on writings from the bronze age, and the population is littered with hateful hypocrites. The Bible is a collection of important historical religious writings that were put together by a set of men, with many other contemporary sources of the time being left out. and we don't really have historical records of why they made what decisions. And if God inspired the writings then, why could there not be more valid works inspired by God in modern times which would be more relevant than those written 2000 years ago. What would be the difference between what's written in the Bible and something written now? It's still written and decided on humans in the end.

Young people today are still plenty spiritual, but they see the corruption in churches, the hypocrisy of what the church teaches vs how the followers behave and live their lives. People who piss on Christianity see no reason to become a part of these corrupted, sometimes hateful, and hypocritical systems when it's perfectly possible to live a moral and Loving and compassionate life without organized religion. Just so you know, I also believe in God or a One higher power, but I also connect with most of such feelings about the current state of organized religions.