Let me recap the history. We used to have:
This question does not appear to be about teaching mathematics, within the scope defined in the help center.
Then, we added the "custom" one:
This question is off-topic because it is a mathematical question as contrasted with a question about mathematics education. For a Stack Exchange site for mathematical questions please see Mathematics.
This made the first one disappear. (Technically it is still there as so-called "default" reason, but it cannot be selected directly.) Originally, this was an oversight; I did not realize that the "default" one would disappear when adding a "custom" one. My first impulse was to add it back as "custom" one (which is basically what is proposed), but then I thought about it, and decided to let it like that for some time and now I am a lot happier without the first reason, here is why:
The "other" option covers it and it is better it is used instead. For the moment I see no reason for having a "general" reason. (Actually, I cannot imagine any circumstances for reinstating the "default"; if anything there should be something more specific.)
For those that do not vote (a lot) what does "other" offer:
It gives a box for writing an explanatory comment. The text "I'm voting to close this question as off-topic because " is already there, but one does not have to start like this and can write whatever comment.
Thus, what this does is that it somehow "forces" the first voter to write a comment. Later voters can select that reason directly, and this will auto-upvote the comment. When closure happens the "default" reason will be displayed, too.
Why this is useful:
When done in a reasonable way it gives more to the point information than a "general" closing message, and does so right away. If somebody intends to supplement the "general" vote by a comment (which is what was considered good practice before the new close-tools were available), then it is near-identical except that "other" is still better (see below).
The reasons used to close as "other" are tracked under the Close Stats accessible to users with access to moderation tools (that is 2k+ here, and 10k+ on graduated sites). Thus, when we use other for some time, we will get over time actual data on what type of "custom" reason we might need.
The in my opinion right way to think about the matter is. The standard way to close is via explaining the specific reason (that is using "other"), only for some standard-scenarios that occur frequently there is a prepared reason.
I do appreciate that voting via "other" is slightly more work. However, on this site we closed only a total of 7 questions over the last 30 days. So, about one question every four days. I thus think that streamlining the vote-to-close process is not really necessary.
Furthermore, as Benjamin Dickman remarks, there is no need to write lengthy explanations in clear cases. If somebody asks about zebras just complete "I'm voting to close this question as off-topic because " by "it is not about math education."
If over time it should turn out we get plenty of blatantly off-topic questions, then we can consider adding a "custom" reason for this.
But at the moment we do not, so we deal with it in the standard way that is by "other."
There is another aspect, namely, that if one votes "other" one is exposed right away as voter, which one might want to avoid in some cases. However, with OPs that seem so problematic one might want to avoid direct interaction there is not rarely anyway still another option that fits ("unclear" or "opinion base" usually) or flag the post for moderators.
To sum this up: I think it is better we do not have a reason like:
This question does not appear to be about teaching mathematics, within the scope defined in the help center.
that merely says it is off-topic because it is not on-topic and can have as a side-effect not well-explained closures. Instead, please, use "other" and explain the specific reason.
If there are proposals for specific new "custom" reasons we can discuss about them. However, as long as we close only one question every three or four days, I do not really see the need either.