We're only allowed three custom close reasons*, which will all necessarily be sub-reasons of "off topic." So if we were to add another custom close reason for a lack of sufficient preparation (or whatever we would call it), it would have to replace one of the existing reasons. I don't dispute that this would probably be a good close reason to have, but the question is, is it more important than the ones we have there right now?
The current close reasons - homework-like, non-mainstream, and engineering - were chosen because, roughly speaking, we (mods and others who offered input) thought they were the three most common types of questions we wanted to most actively discourage. Or more precisely:
- they are used reasonably often
- it's useful to have specific, standardized wording for them
- we want to actively remind people that questions of that nature are not on topic
- they tend not to overlap
Perhaps lack of preparation meets these criteria better than one of the current reasons, but I'm not entirely convinced that it does. In particular, I think a lot of these questions also fall under homework-like (including the example you linked to) or too broad, and can be closed as such. For those that don't, you can always type in a custom close reason which will be posted as a comment. And I think that ability is sufficient to handle these sorts of questions. I could be wrong, though.
I could definitely see myself changing my mind if someone can come up with a wording for the new close reason that would be reasonably objective.
*actually it is technically possible to have up to 5, but all sites other than Stack Overflow are limited to 3 and we would have to petition the SE team with a very strong case to get them to increase our allotment.
On a separate note, I always downvote questions that show a lack of appropriate preparation. This relates to the issue in this meta question: I consider this site to be for expert-level questions in the sense that an asker must have adequate preparation in physics to coherently formulate their question and understand the answer they are seeking. Questions that don't satisfy that criterion demonstrate a lack of research effort (in a sense) and are sometimes also unclear, which is the criterion to cast a downvote.