I think that the question is too broad for IPS.SE.
I'll second what Rainbacon said regarding the relationship between the question and the help center's current contents, but I have a couple of additional observations:
1. "Why" is often an extremely difficult question to answer.
The glib answer in the case of the question linked in your post here would be along the lines of "it's what most people do most of the time, and is so common that someone not making eye contact seems strange". That doesn't explain why the behavior exists, but tautologically states that people want it because that's what they want. Not very helpful, and probably not the sort of answer you were looking for. It's also not really supported by anything-- it's an assertion about what "everyone" thinks, and not really amenable to the "back it up" policy.
If you're looking for a description of the genesis of that behavioral preference in humans (which I assume is closer to what you wanted) I don't know how someone might answer that question in a definitive way. It's like asking what the purpose of the human hand typically having five digits is. It's not clear that there is a specific purpose to it.
This is very different from your counterexample of "how is electricity produced?". This is a physical process which is well-understood, and that understanding is applied every moment of every day as people build and operate equipment designed to execute that process in order to make electricity.
2. The answer, if one truly exists and is provided, doesn't seem related to interpersonal skills.
I imagine that the intent behind a "why" question like this is often to help someone understand a norm that they have trouble complying with naturally. And an understanding of the purpose behind that norm might help someone "get it" and then start participating in the "normal" mode, or help them compensate with other behaviors that address the same rationale but are easier for that person to do.
A goal like that would be better addressed by a more focused question, like "I have trouble making an appropriate amount of eye contact during conversation, which puts people off. How can I make them feel more at ease when speaking with me?". This includes the problem and the goal. The "why" version of the question includes neither.
3. It's unclear what a "good" or "bad" answer would be.
In the "how is electricity made" example, you state that there is "no need to go into all the tiny little details". But how would an answerer know which details to include without any other context? Is "sometimes, steam spins spins a turbine" enough? Or does the asker need a description of how electricity is produced in a coal plant vs. a nuclear plant vs. a photovoltaic cell? Or is the asker really looking for a description of how electrons are made to travel across metal wires, and how those electrons power devices? The question is unclear as to which of those would be preferred.
And these are for a well-defined, well-understood, physical process. For an essentially ubiquitous behavioral norm. "Why" questions about self-reinforcing behavioral norms seem almost guaranteed to stray into pseudo-science, abstract philosophy, and sets of assertions and assumptions that the Q&A format of StackExchange can't really accommodate.