The problem is that just looking through the queue of unanswered questions takes effort.
If I have a list of good, honestly-asked questions, I'm willing to look for a few that I know how to answer and answer them. If I have a vast pile of "plz do this homework for me [transcribed question with errors or link to a private PDF]" then I just won't use Stack Overflow.
All SO tags I've ever followed the number of people answering is greater than the people asking, so that's not really an issue, I think. Although, maybe, it felt that way due to down voting.
The case you described is easy to identify and you can just leave a comment, which will be useful to both: other people answering questions and the person asking. Different from the down vote.
If I have a list of good, honestly-asked questions, I'm willing to look for a few that I know how to answer and answer them. If I have a vast pile of "plz do this homework for me [transcribed question with errors or link to a private PDF]" then I just won't use Stack Overflow.