In a way I am of a different opinion than orthocresol, I agree with the closure; here's my take.
Even after reading the question again and again, then reading all the answers, and reading the question again, I still cannot really figure out what you actually want to know; something I already pointed out in a comment. To me the question boils down to: why did enzymes evolve? That is my own personal opinion and take-away from it, and precisely for that reason I have left the comment. And in that context I agree that it is not focused enough.
Often such questions (in the context how I perceived it) might be closed as duplicates of Resources for learning Chemistry, as a standard textbook may do a better job in explaining the fundamental concepts. You are very welcome to ask questions to clarify these concepts.
Maybe I am too hung up on the wording. I just cannot understand what you mean with the following:
Given that the probability of three molecules coexisting and colliding is less than two, [...]
The probability of three molecules coexisting is close to one; all probabilities are at most one, so I don't understand what you mean with 'less than two'.
In an enzymatic reaction there is no collision between three molecules to begin with; I cannot think of any reaction, where three molecules collide to form a product, except for a few gas phase model reactions.
[...] I’d have thought that evolution would have selected for “simpler” pathways.
I just don't understand what you mean with simpler. Just because a molecule is more complex, it doesn't mean the pathway has to be more complex, too. Like an electric screwdriver is a lot more complex than a regular one, but the process of using the tool is similar, and the same result is achieved faster, and I would argue much simpler.
However, I am just one person, and, as is evident, there are some people who understood your question perfectly fine. Even as well as that they posted an answer to it. It is not the first time where this happens, and it will not be the last. That is why we have majority decisions and meta discussions.
Now back to meta:
With all due respect, what could possibly be the point of choking off inquiry in an area of knowledge where curiosity is typically celebrated?
First of all, nothing is set in stone here. Your question has been closed to allow it to be improved. It wasn't deleted, it doesn't even have a negative score. So I would say your inquiry wasn't choked off.
The Stack Exchange model is to be used as somewhat of a last resort. The more research went into asking a question, the better they are received by the community.
Chemistry.se doesn't aim to replicate content that is available in a more concise form somewhere else. It really is more for the hard to find stuff.
So while we do encourage curiosity, and sometimes even celebrate it, we also would like to make a strong case for self-study. If you are really curious, you have to help along with putting in some work yourself.
Why not err on the side of “clearly there are people that understand the question, so why not let people decide for themselves if the question is to broad?”
I am all for erring on the side of letting a question open. I have often voiced my concern that some questions get shut down prematurely.
Broad questions often attract broad answers, they often attract follow up questions, which will often be posted as answers by users unfamiliar with our site. All of which has to be dealt with down the line. If a question can be improved in a way that these things may be avoided, especially while all participants are still active, I am all for it.
I am sure your meta post will lead to some of our community members reconsider your question; maybe you'd even consider rewording it a bit. If all of that leads to a better findable and understandable Q&A, I'd say the system works fine.
meta-meta: If you want to notify the participants in the comments, prepend
@ to their display name.