Why can't anyone write a correctly spelled sentence on here?

There's really no reason for incorrectly spelled "words" (entire sentences don't normally have standard spelling) because Yahoo has a spell checker that underlines incorrect spellings. My personal opinion is that spell checkers have contributed to the decrease in the ability to spell correctly by memory. Texting with it's phonetic and other types of spelling are also partly to blame. As to people with English as a second language, first of all I doubt that those who claim a large component of those people on this forum really have no more than a very few data points to "prove" their claim. Secondly, all those people need do is correct the spelling flagged by the spell checker.

Because one of the reasons they are asking questions on here is to help them to improve their English.  Could you even speak their first language to the same standard? Don't worry about trolls and adolescent texters; they'll soon grow out of it.

Because too many native speakers "could careless" about "there" spelling, "grammer" and semantics.

Some contributors to this website do not speak English as their first language.

It's a combination of problems; uneducated hayseeds, recent adolescents who have grown up with texting instead of book-reading, the undiagnosed learning- disabled, downright laziness and/or stupidity, and trolling. At this point I'd estimate that 80-85% of Y!A posts have one or more substantial spelling, grammar or punctuation errors. 

I think, sadly, the wholly or even mostly literate are a dying breed. 

We have a few generations of poorly educated people.