Oh that's a good one!
I've worked with some truly incredible teachers putting in 60-70+ hour weeks during term to try and get their exam classes over the line. I think there must be a few factors at play.
Firstly, assuming we're talking about the GCSE (not IGCSE) - the exams are very hard. English has a very low grade 9 percentage compared to other subjects like History, Geography or Science. You can see this here: https://www.bstubbs.co.uk/gcse.htm
English Language has 2.2% of grades nationally achieving grade 9, with 819199 entries. History has 5.8% of grades nationally achieving grade 9, with 289677 entries.
Partly you might explain this away because History is an optional subject, so students choose to take it who are good at it, whilst in English, everyone has to take it, so the spread of grades is greater. But even Maths has 3.4% of grades nationally getting a 9, with 851268 entries, so I don't quite buy this.
I think partly then it's due to the reforms in 2013-2014 where the government decided to deliberately make English and Maths hard, because there was the perception they had become too easy.
Then you have reading habits. The PISA scores have come out recently, which show England is doing fairly well compared to other countries when it comes to reading. Nonetheless, there are a lot of studies showing that reading - especially of long texts - is in decline. You can pin this variously on modern life, social media, videogames, Youtube, homework, parents working longer hours etc. But what is undeniable is that reading is on a downward trend - and to do well in English, you do need to be reading regularly, even if that's the newspaper, a popular science book or a biography of a cricketer, and not War and Peace!
Finally, you have the marking. I'll link an article below, but the marking of English exams is highly variable. You would expect this with an interpretive subject, but nonetheless, the margin of error is still very high (compared to say Physics)
https://mrsalles.substack.com/p/how-to-write-grade-9-essay-including
There are other factors too, but these are the ones that come to mind. I would say that nobody (schools, teachers, students or parents) are to blame here - it's a really complex web of politics and culture.