Often when I ask a question, the tiniest mistakes in it will get edited out almost immediately, sometimes entire sentences will be reformulated in a better way. I am assuming that's done by actual people.
What is the reasoning behind allocating presumably a lot of resources towards such a meticulous task, especially given the large number of questions asked on this website?
In short: Because we care, and because we're incentivized to do so.
Stack Exchange's stated goal is to be a repository of high-quality information in Q&A format. Just have a look at Stack Overflow's Tour page, which says:
With your help, we're working together to build a library of detailed, high-quality answers to every question about programming.
Stack Exchange also encourages editing a lot. Users who do not have the ability to edit unilaterally will get 2 reputation for each edit that's approved, and there are a bunch of badges related to editing. You get Editor for your first edit, Cleanup for your first rollback, Tag editor for your first tag wiki edit, Explainer for editing a question (hopefully to be better) and answering it, and many more that are "evolutions" of those badges.
All of these incentives promote users to not just edit posts, but to edit them well so that their edit suggestions are approved and they get some reputation in return. Once they can edit unilaterally, there are a bunch of badges that continue to incentivize them to edit questions to be the best they can be. Even without that incentive, many have come to Stack Exchange seeking knowledge and feel a sense of accomplishment in giving back to the site. The more they give back, the more the system empowers them with privileges to continue giving back in newer and better ways.
