why is there not flag option for it, like on other sites?
On other sites, particularly sites with more traffic, the rate at which such questions are asked may exceed the ability of the community to effectively moderate using the default tools available. This creates a practical need for customized tools to make moderating a particular type of question more efficient.
On our site, no such need has been shown. I don't see resource-hunting questions sitting around for days, staying open and attracting a bunch of answers, misleading readers into thinking they are on-topic. I also don't see any significant number of users coming on to Meta and asking why their questions were closed because they couldn't figure it out on their own.
If there is no close reason for particular type of questions, they cannot be truly deemed off topic.
This is not correct. Meta discussions and the Help Center are what define the scope of the site. Not close reasons.
Close reasons have one purpose: Provide guidance to authors of closed questions to help them understand why users voted to close and what they can do about it. The current default off-topic close reason is:
This question does not appear to be about engineering, within the scope defined in the help center.
The hope is that users who see this message under their closed questions will go and look for more information in the help center. The default off-topic close reason can be customized by moderators; unless anyone objects, I think it would make sense to link straight to help/on-topic rather than (or in addition to) linking them to the main page of the help center.