Hi mteye,

It's not a stupid question.

Are you dealing with jerks?

May be, but to me that doesn't change the reason people don't always share information with others.

There may be various reasons people hold off on sharing what they know or think they know about about others.

Too sensitive topic:
Sometime the information can be a source of dispute, or emotional upset for you, so people prefer avoiding the conversation, because they are not ready (or don't want to), or think your are not ready, to deal with the possible emotional outcome. 

Too personal topic:
likewise, sometime the information is too intimate, and people are not confortable talking about it.

Not the right time:
Sometime people don't have the time to talk about an important topic at that given time. Sometime, people may think about that information, but won't interrupt the current activity, discussion or else, because it would create an interruption in the flow of activity.

Not interested in sharing:
Sometime people just don't care to know what you think about what they know about you.

Not the right person to tell you:
Sometime, people don't want to overstep boundaries, and won't tell you what they know about you, because it is not their responsibility or sharing this information would cause them trouble with someone else.

They don't know you would like to know:
Just like that. Some people don't realize that you may actually not know what they know, and that you would like to know.

Others?

As for your examples...
Maybe your therapist may have felt you were not ready to know, or thought it was not the right time, or didn't want to deal with the diagnosis a that time. Who knows...
Those you thought wer your friends, maybe were trying to protect you, or didn't want to have to deal with the consequences of the topic... 

There are times, it's just hard to break a news to someone... because you don't want to hurt them, or you don't want them to hurt you....

My rule of thumb is that you tell people what you expect from them. No guessing game. 
If you want people to be open and honest with you, then, let them know that is how you operate. Tell them that honesty is something you greatly appreciate, even if sometime it may be painful to hear, that you would be grateful they let you know rather than not.

I tried to cover several possible situations. I hope this does provides some answers. Let me know if you need any clarification.