Lets say that it takes 15 minutes in chat to help a person (who wants to be helped in asking a good question rather than getting the answer now).
Next, lets apply Sturgeon's law to Stack Overflow. There are 8000 questions a day and 90% of them are crap. Thats 7200 questions that need work. This is 1,800 hours of mentoring per day.
The close (and down vote) is such a minimal amount of work that it allows the group within the site that is striving for a particular vision of quality that it allows them to do the most they can. Furthermore, it is not infrequent that a person who provides assistant for trying to unravel a question from getting a "why can't you just help me now?" with assorted vulgarity interspersed.
On smaller sites, with a greatly reduced amount of questions per day the group capable of mentoring as well as moderating is able to spend the 15 minutes of time without significantly impacting the time taken to moderate and curate the rest of the questions.
With 8000 questions per day, its really hard to look at all of them. As I write this, there are only 16 people who have exhausted their close reviews for the day ( https://stackoverflow.com/review/close/stats - only 16 have 40 reviews). That doesn't show the people who are doing new questions, but it does give an idea of how shallow the bench of people who are spending time to moderate the site actually is.
Getting more people to help out would be great, but most people aren't doing anything to help.
There are also people who believe that up voting a newbie question and saying "welcome to the site" in the comments without improving the question is helpful.
The first post review and triage queues are intended for providing this help. You can see that this doesn't always work well ( https://stackoverflow.com/review/first-posts/19570266 or https://stackoverflow.com/review/first-posts/19569647 - note the not even trying to fix the grammar of the question or asking for the necessary information or providing help on how to improve the question, just no action needed)