I was asking why one thinks this happens. I know why it upsets me
Oh, it upsets me, too.
When I was teaching, and grading essays, grammatical (and spelling) mistakes drove me nuts; these were university students with good grades. However, my professor used to argue that I was supposed to be grading their political knowledge, not their grammar and to ignore the latter.
I have also worked as an editor (in the parliamentary debates office) for a year, which, if anything increased my intolerance on such matters.
However, to your question:
1: I think that nowadays people read less, in general.
Reading teaches how to construct sentences and paragraphs, and also teaches you how to express yourself on paper. Actually, copious reading can bestow an ease and facility with the written word. You learn this - these skills - almost without realising it, if you are immersed in books and good newspapers, periodicals, publications.
But, knowing how to read doesn't mean that you are at ease when writing; as with any other activity, this takes practice, and you need to do it a lot.
2. The online world has encouraged the use of casual language and slang and abbreviations; originally, this was for reasons of cost - in the early days of online technology, you paid to send texts, and thus, famously, people started compressing words to fit into the number of words allowed and thereby not incur whatever tariffs were imposed.
Online etiquette - and language - have not kept pace with the tech revolution; I daresay it will develop or evolve over time, but has yet to do so.
3. Spell-check: I have lost count of the number of occasions where the computer has decided to substitute what it thinks I wanted to write, or should have written, for what I actually wrote.
And yes, to my horror - and I do know the difference between "quite and quiet", and the classic "there/their/they're" - I've castigated students who got this wrong - posts or emails (private ones - professional ones, I do double check such things) have sometimes appeared with a word I had not - not only not intended to write, but was pretty certain I hadn't written until the computer, in its wisdom, decided to over-ride what I had thought I had written.
Surely at some point in their lives they knew how to use one. Can one really forget this or do you think they never really learned? I was completely blindsided when I saw this from the smartest guy in my high school class-- and it wasn't a typo. He did this at least 2 years in a row.
No, there are individuals who have never learned the differences between these words, and never troubled to master these things, not considering them to be of importance.
