This is because French is a ridiculously overaged language, with a fuckton of exceptions, double entendre (hé) and other flaws.
To take an example, the word used to say "more" is "plus". And the word used to say "no more" is also "plus". Yep. Although to be honest, they haven't the same pronunciation, but it still can cause headache.. for instance, some time ago I was reading a poster resuming the opinions of a political party, and there was a line : "Plus de prison.". For any other party, I would have though immediately "More jails !" (French prisons are crowded) , but this was a far-left party, so the "No more jail" option wasn't unrealistic also.
So really, French might be a great language for literacy, but for everyday use it sucks. There's like a dozen of tenses, half a dozen of moods, and that's a miracle we don't have some sort of declension too. There's plenty of double letters or other silent letters, and grammar rules take years to understand and maybe a decade to master. No wonder why almost no one is able to write it correctly.
Oh, and also, some people might struggle with "there / their / they're", but you'll be appalled by the number of people who have trouble choosing correctly between "est / er / ai / ait / ais / aie / aient / et / é / è / ê / ë / ae / es / ..." simply because they all sound the same (minus some subtlety) but must be placed according some bizzare grammatical rule.