I don't think they're particularly "hard" as such. The kids have been learning for several hours per day for 5 years at secondary by the time they sit their exams. That's a lot of hours of teaching!
Yes, there's a lot of content that can potentially crop up in the exam, and lots of what the kids are taught doesn't appear. Perhaps the scope/breadth could be reduced, but the difficulty remains or increases so that then the exams have questions on a narrower range of knowledge, especially in, say, humanities subjects and perhaps English Lit, where the current breadth is quite ridiculous.
It's no surprise that the OP, as a parent, takes a quick look at an exam paper and thinks it's hard - they've not had the lessons, practice tests, revision sessions, etc, that the kids have had. Even if they'd done the subjects themselves 20/30 years ago, they'd have forgotten most of it and the syllabus will have changed so some of what parents learned for their GCSEs may have been removed and replaced with new topics.
I do think that teaching practices could be improved to help pupils though. My DS's school were known for their excellent history results at GCSE and A level. We were very impressed by the history lessons from literally the first week in year 7. They started with teaching bias, sources, intepretation, etc right from the first lesson, treated more importantly that content and learning of dates/facts, etc. They followed that through the school - barely any emphasis at all on learning dates/facts and constant practice on sources, i.e. diaries, illustrations, inscriptions, comparing one commentary of an event against another, etc., and also on the other "themes" of history exams such as cause & consequence etc. It was only in years 10 and 11 when they started learning "facts" as required for the looming GCSE exam, but they'd all got the core "skills" by that time that reading and understanding about the events to be examined was basically all they were doing, as they didn't need to learn exam technique, answering questions, etc that had been done to death in earlier years. It was a completely different way of teaching compared with my school days where it was all facts for the earlier years and then exam technique etc crammed in to the last few weeks before the exam!