Why don't high-ranking journals go solo?
Many journals are publishers' own products, "going solo" makes no sense. It's like asking why Gmail does not segregate from Google. In some cases, professional societies hire publishers to take care of their official publishing organ, I imagine your question relates to these cases. Otherwise, there have been cases of the entire editorial board leaving for another journal, I've heard it happened to a Frontiers journal over concerns on quality, but in this case the journal is still there, just edited by other people.

Running a professional-level journal is not trivial and there are reasons to favor established organizations, commercial or not, to handle that part. These include administration, secure web hosting, a long-term back-up strategy, typesetting, distribution, submitting accurate and complete article metadata to third parties like Pubmed or Web of Science, printing when applicable, etc. and of course finding the money to do all of these. 

For many editors, it's a pretty straightforward decision to outsource these hurdles to a specialized organization, be it for profit or not, especially if there is an ongoing issue-free relationship going on for years.