Often it's ignorance and/or laziness. Grammar and sentence structure hasn't been taught in schools for years. Spellcheck only fixes errors in spelling (well, sometimes) but not errors in comprehension. God forbid anyone should actually look up a word in an actual dictionary - well they can't, because they haven't a clue how to spell either. 'Shammy' written in the paper rather than 'chamois' springs to mind as a particularly fine example.
Probably given the same spiel many times now.
I'm not sure about all of you who may see this through the eyes of your kids who are going through the current school system, but for me, sentence structure and grammar was not taught as a matter of theory. It was more something that was taught subtly as part of other language assessment, e.g. spelling tests, writing essays. Occasionally in primary school, we had punctuation drills and parts of speech identification quizzes.
In high school, however, the English curriculum is centred around genre and devices. For example, brochures, campaigns, advertising, English literature, humour, crime, literary critique, letter writing, story plots. Again, there is no specific instruction in grammar and sentence structure. You were certainly penalised in your assessment if grammar and sentence structure didn't hit the mark, but often you would learn about this stuff by either (a) asking the teacher when pulled up in class (hopefully before an exam), or (b) using a self-directed learning book which helped to identify and rectify common English errors in grammar.
The interesting bit about dictionaries is that it was flagged to us when we were younger in that it was supposed to
help us spell, which I found weird as you can't find a word if you don't know how to spell it (it might have been OK if your guess was not far off, but if you're way off track you had no chance). Apart from that, dictionaries still have a place - so do thesauruses - but trust if anyone knows how to use them
properly. Another thing not taught well in school - the subtlety between synonyms and why this is important.
These days, outside of academia and some professions where written communication is critical, the general public - at least in Australia - doesn't care really about your attention to competency in numeracy and literacy. As long as you can string something to be understood, refinement is unimportant. Ever been branded a Grammar naz_ before? How many people actually care about being competent in maths, especially when calculators are now readily available? I think this is reflected in the curriculum - society would rather see people who can throw words together in a variety of situations, rather than care about how accurately they can do this. (There is merit to this argument - it is no good learning about sentence structure theory - simple vs compound, subjects and predicates, and so on - if one doesn't also understand the subtleties of how to compose a report, a news article, a story with a plot, etc.)
Is it a sad testament to our society?
For what it's worth, I'm only glad for spell check because, 95% of the time, it picks up on my silly typos. But for most publications, it is re-read again carefully for spelling errors. I don't run the spell check machine. I also don't use the grammar checker.
Why is it when you are down everyone lines up for their turn to put the boot in?
So you don't feel more than one boot on your body at a time.