Many times on this forum I see questions edited, e.g. just because one single letter gets corrected from a little to a capital letter. Almost all across the internet nobody really cares about spelling, as long as it's still clear to everyone what was meant.
Now I do understand that the situation here is a bit different, since this is a forum about a language. But I still think some users here are rather willing to correct other people than to actually help them.
I think you are overestimating the effort users put into performing spell checks. I would be surprised if any of us browsed the site for no other reason than finding and correcting spelling mistakes. Rather, we read content because we are interested in it and edit spelling mistakes that we happen to find in the process, which hardly costs any time at all.
When during reading a post I come to the conclusion that it’s worth editing¹, I open an edit window in another tab, in which I fix all spelling mistakes I find while reading it. This takes almost no additional time.
Also note that our question and answers are not only intended for the asker but to be a permanent repository. As of now, we have 73 question with more than 10000 views. This makes fixing spelling mistakes more important and economic than it may be in many other places on the Internet.
¹ taking account the severity of the spelling mistakes, the importance of the question and whether the post would be bumped to the front page or is already there
In addition to Wrzlprmft's answer: There are users, who are feeling disturbed by spelling errors - let's call it some mild form of OCD. Why would you disallow them to improve those on their own time, if it (a) makes them feel better, and (b) improves the average quality of the site?
For example, I do loads of code reviews and proof-reading at work, and a lot of error categories jump out at me from all kinds of texts. I feel distracted from the contents of texts containing spelling mistakes and I'm less likely to produce good contributions to them. It's a relief that SE offers possibilities to get rid of these, while e.g. in newspapers, I can do nothing about them.
