That's true, but kinda rude if I may say :O

Dude I'm so aware of that, and totally not imposing a soul to leave a comment.

There might be several reasons.

There is no rule that everyone who reads a post must leave a comment. People are free to express their opinions or keep quiet.

I have very high standards for English writing.  It takes a lot of work to correct even as small a passage as 2-3 paragraphs.  I might spend 10-15 minutes doing it and, even then, that's just a first pass.  If the quality of the English text is really abysmal, I just won't feel motivated to make the effort because I would have to literally rewrite the text and, frankly, sometimes I don't completely understand what the author is trying to say.  

That notwithstanding, I have done some major rewriting on some notebook entries.  Hey, you're not paying me so I think I might have the right to pass on the marvelous opportunity to correct your notebook entry or to answer your question.

I might also look at a notebook entry or a question and decide that I don't have enough knowledge to answer the question.  I might also feel that the question is so easy to answer that I'll let someone else take a whack at it.  I try to answer questions where I feel I can make a contribution that might not be brought up by others.

See.. it's already taken me several minutes to write this much.  I only have so many minutes in the day to devote to italki.

Maybe they have nothing to say on this topic? Or in such a bad mood :)

Hi! Maybe you're tired, you had a bad day, you hate the person who is asking or you just don't want to answer. So many reasons as people. But something is something, that person has read the question and has showed a bit of interest. You can't have everything! Learning to live with our differences is essential! Have a nice day! :D