Yes, I have the same feeling at work and here is my understanding of the points listed:

Q1. Don't directly address the point of your question / provide a much complex answer (or even worse, a non-answer) to a simple question.

A1. This comes down to a lack/abundance of knowledge and/or how to handle a specific situation. If someone is not directly addressing the point of your question, they may not have the knowledge to answer and therefor may feel uncomfortable because now they don't know how to handle the situation they are put in. They have a sense of pride and they don't want to look like they know less than you or others around them.

A good example is asking someone something personal in a topic that you may excel at and they may lack.
On the other hand, it can be the opposite. The person asking the question may view the other person answering with an over complicated answer. In reality, it is the person asking the question who has little knowledge and the person answering has greater knowledge.

Another good example is telling people what I do for a living since it is in the semi-conductor field. I have only had two people, not including family, wanting me to explain more about what I do. For others, if I try to explain beyond saying "I work on photo lithography equipment," they view it as an over complicated answer.

Q2. Don't stay focused to the point of the discussion

A2. This comes down to caring about the subject at hand or scope of knowledge and divided attention. I think if you are in a discussion and the other person starts losing focus then you may have exceeded their knowledge base in said discussion and they don't care to continue. On the other hand, they may simply not be giving you their full attention in the first place which will lead to de-focusing on the discussion.

A good example is simply what you and the others around you have in common and if you think on the same wave length. If people don't have some similarities or they don't think the same way, they won't stay too focused in a discussion.

Q3. Don't have some level of clarity in their train of thought and speech
A3. I think this comes down to two things: 1.) How well and how comfortable is that individual with articulating their thoughts and emotions and 2.) How comfortable are they with their knowledge base in that discussion.
This can be a cultural thing, a language barrier, a feeling of confidence in ones self and the topic

Q4. Generally over-complicate matters by wandering off to other related subjects and extending the scope of the discussion
A4. So this one is easy because I think we all have encountered someone like this in our lives and if you haven't I can explain. If someone is over-complicating matters by wandering off to other related subjects and extending the scope of the discussion, it is because they may have basic knowledge on the discussion and since they don't want to look stupid, they wander off on to other related subjects to make it look like they have a broad scope of knowledge.

I work with someone who is exactly like this. The first time I worked with him, we were talking about interferometers (very accurate LASERs for measuring), and boy, hehe, that conversation just went from one "related" topic to another. Anytime I try to ask something in depth on the topics he brought up he gets upset and veers. At first, I though he was very knowledgeable, but I later found out this is a trick he pulls as a dog and pony show to "show how smart he is." In reality, he has very little life experience with those other topics yet he talks it up like he knows so much. Long story short, he constantly always goes behind are backs at work, he's untrustworthy, pain in the ass to work with, but stays employed since he knows how to work on our systems very well. This person has an extreme sense of pride and doesn't want people to see his mistakes.

In conclusion, why do some people not communicate clearly? They either can't or don't want to due to some kind of motive they have which may either because they are simply trying to hide something, or they don't know and don't want you to know, or it may simply be a cultural or mental inability. In our case, it's the humble engineer interacting with other people who may not be so humble. You are not alone and I am glad I am not alone. I feel this way time-and-time again through my professional career and personal life.

Thanks for bringing up this topic up. It was good to talk about it with other who feel the same way.