I think that your question needs to be answered in two parts:
- Why is the question highly rated?
- Why does it not refer to open questions in mathematics?
I shall answer those in reverse order.
Why does it not refer to open questions?
I think that you ask this because of a misunderstanding: the question is specifically about beginners’ mistakes (for some advanced value of “beginners”), which they abandon “when their mistake is pointed out”. It is, therefore, not about conjectures which were popularly believed and then disproved; nor is it about conjectures which may yet be disproved.
N.B. The odd answer mentions a misconception that something is an open question!
Why is still a good question (despite not referring to open questions) is the subject of the next part.
Why is the question highly rated?
The questioner says they are interested in “beliefs many intelligent people have while learning mathematics, … and … why they have these beliefs”. They also call it “more like a psychological question than a mathematical one” and ask for cases from “reasonably advanced mathematics” where “the reasons they are found plausible are quite varied”.
As mentioned in comments, knowing of these misconceptions, how they arise and how they may be shown wrong can be helpful:
- To avoid comparable pitfalls – in some cases the very same one – in one’s own thinking.
- To understand how one’s pupils go wrong and to set them right again.
I think, moreover, that people sometimes find it entertaining and even satisfying to look back on their own and others’ mistakes because:
- It reminds them how far they have come.
- Spotting an error is a sort of problem-solving.
- Identifying the point at which an argument fails may improve one’s understanding.
- The flash of understanding is comparable to the delighted surprise when one understands a joke; more so because of the erroneous twist than when one finds or follows a valid proof.