Lacking Grammatical Skills...

I'm really not trying to be insulting, this is just a question that has been bothering me...

I'm just wondering... why do some people post stories with horrible grammar? Not to be insulting, but doesn't that take away from the story. and I'm not saying I haven't done it either! but my current projects I have been very sure to make it as grammatically correct as possible.

I just want to know now... why do people not use contractions, proper punctuation and so on? Is it really that hard?

your (your eyes are pretty)
you're (you are)

their (their dog ran away)
they're (they are)
there (you can sit there)

I'm kind of at the end of my rope. I mean, don't most people have spell and grammar check at their disposal?

My problem is that I think it takes away from a story's quality. People struggle to understand what a writer is trying to convey and still the writer won't attempt to fix it!

Please explain!