So whatwouldactually fix the problem? I can see that you're right but in some way it's quite mind-boggling really.
Nothing will fix the problem. And it's a problem that's always existed. Imo some of it is down to how individual people's brains and memories work. I know that's an unfashionable view, and admittedly some of how their brains work will be down to input and influence in very early childhood. Reading a lot helps, but it's not necessarily enough to create an excellent speller. I'm the child of 'wordy', cryptic crossword fans. Dh was (and remains) a hugely prolific reader as a child, and is very bright, but has non-wordy, parents who spell quite poorly. I've been a pretty immaculate speller since an early age. Dh is... ok at spelling. Oh and the internet simultaneously doesn't help and makes poor spelling and grammar much more visible to a huge audience.
The problem in school is that a lot of what is taught does not sink in (in the long term or even the short to medium term). I'm wondering why you find it mind-boggling. After all, people are usually quite comfortable admitting that they remember practically none of what they learned in biology, geography or history at school, or that they're rubbish at maths. But for some reason they don't seem to believe that many people have a similar inability to retain spellings and grammatical knowledge.
There's also the myth that only English-speakers have poor spelling and grammar. Or that this is a relatively recent phenomenon. Neither of those things is true! It probably doesn't help that English spelling is batshit crazy, and that teaching grammar was not really done much in British state schools for generations. But bemoaning the state of a people's spelling and grammar is something that has happened throughout the ages, probably in every country in the world. I once read a glossary created by irritated monks, which was a list of 'infuriating SPAG errors everyone is making these days'. In Latin.
In the end, you have to ask yourself how much it really matters in the grand scheme of things. Getting pissed off and judging people achieves nothing. Some people write well, some don't. In most situations, the most important thing is communication.