Indeed. Questions looking for offsite resources don't work very well with this site's format. The site is designed around objective questions where there's a "right answer". Occasionally there can be debate around what the right answer actually is, and sometimes a little subjectivity can be OK, but really the site is optimized for finding and agreeing on the right answer, and putting that at the top where visitors can easily find it.
"Where can I find a book about X?" doesn't fit that format. There's usually a whole list of answers, and (with online resources) they often go out of date quickly. A forum-style site wouldn't care about that: you'd end up with a long thread with several pages of posts, each of which has one person's list. The lists would overlap, and by the time you find the page months after the question was asked, half of the links are dead. Stack Exchange tries to avoid that kind of content.
Do feel free to ask that kind of question on other sites, if you're not afraid of that kind of forum thread. Stack Exchange is a complement to forum sites, not a competitor. But when you have a specific question about Trinity Music Theory - the kind of question you might look up in the books and websites you're asking about - come back to us and ask that question. Those factual questions are exactly the kind of thing we can help you with.