IMHO it has to be the education system in the UK.
It's because secondary English education is dominated by literature, poetry, fiction, etc. The amount of teaching/examination on actual literacy, spelling, grammar, etc., is minimal, basically the least that they can get away with. It's all about interpretation, understanding plots and themes, etc rather than the nitty gritty of real life literacy. Spelling and grammar mistakes are tolerated if you can recite the quote correctly or understand the importance of a sub-plot or the meaning behind a poem.
When I was in secondary in the 70s we spent many, many lessons on letter writing (formal and informal), doing comprehension exercises on real life non fiction (i.e. to prove understanding of a recipe or some instructions or a extract from a text book), preparing balanced arguments, doing exercises on summarising a long piece of writing or expanding a list of bullet points into an essay, etc. Spelling and grammar was treated with the utmost importance throughout all that and you'd lose a grade or two if an otherwise perfect piece of work was littered with technical mistakes.
There was a time that Eng Lang and Eng Lit were two completely different subjects. Back in the 70s we had different teachers and each was it's own distinct subject. Nowadays, it's all blurred and dominated by fiction/poetry etc. That turns off the less academically able, so they don't learn grammar/literacy either. I have been completely aghast at my son's five years of secondary "English" education - just book after book after book, then the only writing they did was creative writing, which is again, basically fiction. Things like balanced arguments, essay writing technique etc were taught in other subjects like History. It's a crazy system.