A question maybe answerable but it might still not fit the criteria of Questions we expect here. Let's break down the Questions you have mentioned:
Has Saudi Arabia ever been invaded? : This question was originally aimed at knowing "Why Saudi Arabia has never been invaded?". Of course the Premise was false and that's what you proved in your answer. However if we ignore that, this would result in primarily opinion based debate with numerous users proposing different hypothesis and disagreeing with each other e.g. difficult terrain, weather, no resources (Until oil was discovered). I reworded the question to "Has Saudi Arabia ever been invaded?". Now this phrasing does not lead to opinion based debate and can be answered with facts, as you did. However, It still violates the policy as it is trivia request and about basic historical facts which can be looked up in one simple google search. The closing reason correctly states:
"Requests for trivia or basic historical facts are off-topic if they
can be easily answered by looking up the relevant topic on
Wikipedia. We're trying to complement common historical references,
not duplicate them."
Before the Advent of Agriculture, Was whole world covered with forests?: I actually agree with you on this one. This should not have been closed as it is about history of Geographical features of the Planet. But I think those who closed it did so because this question is more focused on Environment and requires some knowledge of Environmental sciences, not just basic history. But since it is about history anyways, it should not have been closed.
Why do people refuse to acknowledge the Holocaust?: This should have definitely been closed as this is clearly about behaviors and psychology of people regarding a historical event, rather than a historical event itself. Your own answer is focused on a personal experience in proving the event, it does not answer why do some people deny it ever happened. The closing reason is quite clear that People who are experts in Psychology would be better equipped to answer a question related to human behavior of denial than those on History Stack Exchange:
"Questions on social sciences other than History are off-topic
here, unless they also involve history in some fashion. While
ethics, archaeology, etc. are all connected to history, each field
has their own experts who are better equipped to answer such
Is there any chronology of the repartition of productive capital since -1000 B.C?: This should be closed as this is not only about almost the entire span of Human history but also requests off-site resources such as data compiled about such info. This question is definition of Too-broad. Unless the OP mentions a time period, a geographical region and retracts his resource requests, this should rightfully be closed. The first Close Vote was casted with reason "Unclear what you're asking" which is absurd as it is absolutely clear what the OP is asking. OTOH, It is too broad.
You can argue that you did manage to answer but Pardon me if I say it is not really a great answer(Great here defined as an answer which covers everything asked in the Question in required depth and details). It does not cover the complete human history and historical evolution of all the distinct regions and cultures. Of course doing that is impossible, which is why we have the Too-broad reason to close such questions.
The "Too-broad" close vote reason is very clear:
There are either too many possible answers, or good answers would be
too long for this format. Please add details to narrow the answer
set or to isolate an issue that can be answered in a few paragraphs.
What Can you do?
It is admirable that you try to answer questions no matter what are the flaws in them but I would suggest that first of all you should use your moderation privileges.
- If a Question is badly phrased, edit it out and correct it.
- If a Question contains minor statements which may make them off-topic, prune them out.
- If a Question is very much off-topic and radical editing is required to salvage them, suggest that to the OP. If he does it himself, great. If not, Cast your close vote.
Answer only the questions which are eligible to stay on our Site as per the help-page on accepted topics.
Remember, Just because it can be answered, doesn't mean it should be answered.
But that Question was/could be on-topic?
In that case, you have the reopen Votes.
If the Question could be salvaged into an on-topic question, edit it and cast your reopen vote.
If the Question was wrongly closed, cast your reopen vote and see if rest of the community agrees with you.
Also read: Help page of Close Votes & What type of Questions should not be asked?