I do think it's something to do with modern education. I'm not teacher-bashing, because I do recognise that teachers these days have to adopt the practices as laid on from on high (rather than what would be the best for the children who are there right in front of them on a daily basis).
Things like not marking too many errors in written work, which is appropriate in the first couple of years or so as children get to grips with the vagaries of English spelling, but not so in older years where children really do need to know where they're going wrong.
I'm also not so sure about phonics. That can be a good method to get very young children feeling confident in reading but with a limited vocabulary. English is such a mongrel language and for very many words the pronunciation these days don't match the spelling - the speaking and the spelling seem like two different languages, which is a trap for the unwary unless one learns a whole load of spellings by rote.
I do wonder if people who constantly use the of instead of 've realise they're making a mistake but couldn't be bothered about that. It could be that for them the message is more important than the medium but, snobbish as it might seem, if the medium has such basic errors I'm less inclined to spend time figuring out what the message might be.