In brief: The thread in question fills a certain small niche, as a supplement to the reopen review, and it does that well.
Over stretching that thread to contain almost all questions that seem like or can be seens as a request for reopening or undeletion will on the one hand make it less adapted to fill that niche, and on the other hand might alienate some users for very quite little in return.
What is that niche?
The thread is a supplement to the built-in reviews and allows to draw attention to questions that might fall through the cracks, regardless the built-in tools. It is, in my mind, mainly a tool for frequent users of the site. Looking at the last couple contributions, and from memory, it is mainly used by experienced users for questions that are not their own to post "clear-cut" requests for reopening (or perhaps also un-deletion).
What is a "clear-cut" request?
First, it should be clear that the post really asks for reopening (or undeletion or both); and by clear I mean it should be stated explicitly. Second, there should be a specific reason for the request compatible with the sites standards (for example, closed as unclear but got clarified, or closed as dupe but somewhat different on closer inspection). Yet "Why did this question get closed/deleted?" is not "clear cut" on both counts, and definitely not on the second.
Why should other requests not go into that thread?
This thread is not the local dump for "Please reopen my question" pleas but by contrast a tool for communication mainly from frequent users for frequent users. To clutter it with all kinds of non-viable requests and discussions is not a good idea (as is even alluded to in that thread).
Why it is not a good idea to force/close requests into that thread?
One reason is already given above. Another is from the side of the OP. When one must ask for the reopening of one's question one is likely not in the best of all moods. If then one is faced by some process that smacks of bureaucracy it might not go over that well.
What about the clutter?
Even if no request was posted in that thread it would not increase the volume on meta that much. There are at the time of writing 4360 questions on meta and 196 (non-deleted) answers to that thread. So, if we would not have it and each of the answers as a question, we would have 4555 instead of 4360 , or less than a 5% increase. (I do not know how many answers are deleted, but I do not think that many that it changes substantially the conclusion. Also, fairness dictates to acknowledge the thread does not exists since "forever" but again since long enough that it does not change that much.)
I think the thread should be used by frequent users that post clear-cut reopen and related requests with some frequency, and it can be used by however wants.
By contrast, we should not force casual users that did not use it out of ignorance or some other consideration into using it. (One might point out the existence of the thread for the future, but that's it.)
I feel insisting on the usage of that thread is one more thing that can trip-up a user for quite little return; it makes meta just a little more unwelcoming for good-faith requests. In addition, to harming the quality of that thread.