I'd say the overall consensus would be that the internet is an entirely informal place, thus negating any true -need- for proper spelling, grammar and the like; under the assumption that the receiving person/persons will completely understand. So, instead of utilizing what they have been taught, many do not care to try and, instead, submit what seems to be gibberish.
It's really the "I want to be friends with everyone" effect that I see the most; so they treat everyone like they would treat their friends (sometimes), and type informally. There's also the "I really don't give a crap, I do what I want" idea, so they type how they like to; even if it is atrocious and pathetic. There's an unlimited slew of reasons why some sentences look like they've been chewed up and spit out.
Sadly, we'll
have to tolerate it at one point. We do not have any control over anyone else or their habits/issues. All we can do is suggest they type cohesively, or pass on a friendly reminder; or just not talk with them.
* Sytex shrugs. "Just my two cents."