If something is anonymous, why is it less reliable?

Because the person is not willing to identify themselves, there is no way to determine the likely veracity of what they say. It may not be less reliable than from someone who identifies themselves, but it is not possible to judge, so in my mind, it is unreliable.

From what Ive heard anonymous is known for writing a lot of crap over the years, and he really has just become less reliable on his information. He may seem like he knows a lot, but this is often a stretch because often times he just makes the stuff up.

It isn't automatically less reliable. If it's simply stating something you can check against reliable sources, it's fine. But if it's stating something novel and unproven - for instance, "Wearing a green hat cures the common cold" - identity and reputation of the person stating it become a big factor in assessing the reliability.

Because the contributor isn't putting their reputation at risk- they could submit something that would normally embarrass them but, since it is anonymous, can't be linked back to them

People who are anonymous aren't afraid of ruining their reputation, and can say really stupid things if they want to.

Because you can't look the person up (online, for example) and find out if they really know what they're talking about.

As you don't know the person, they could be biased and they might have a good reason for not wanting to give their name.