They are very touchy subjects. Plain and simple. People tend to be extremely opinionated on these topics, and it causes a disturbance in the general flow of an intelligent conversation when people feel threatened for speaking freely.
These forums evoke certain beliefs out of each of us. These beliefs vary depending on age, sex, race, creed, location, where you grew up, how you grew up, and all the other factors of our lives. So we all tend to discover what works for us. But people have a hard time dealing with the fact that what works for them may not (and likely will not) work for someone else. Thus the conversation about philosophy will turn into an argument. So instead people just shy away from the topics altogether for fear of finding themselves in some rant about whose life plan is right.