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