> who even writes subtitles?
I do, as a hobby. Typesetting in particular is one of the most rewarding things I've ever done. There are few things as satisfying me as being able to set a sign so that it looks like it's part of the video.
If you're curious, there's a tool out there called Aegisub. Pretty much every fansubber uses it, and it can handle most parts of the process: timing, typesetting, TL/editing, and it shows you a preview of what it looks like muxed with the video (though you do need a separate tool to actually generate muxed MKV files).
> Both are in English, so I don't see why they can't just transcribe what's being said. Instead, for dialogues such as "The greatest trick the Devil ever pulled was convincing the world he didn't exist.", the subtitle is written as "The devil tricked the world into thinking he didn't exist."
You have entered the wonderful world of Hong Kong bootlegs, which are usually just called "HKs". The people who work on these just plain don't care. They translate the movie into Chinese, and then for whatever other language they release subtitles in, they just Google Translate their Chinese subs into that language, even if it's the movie's original language.
Fansubbers hate them, and if the show or movie in question was never officially released in English, there are some groups who will clean up these subtitles to make them more presentable and fixing the godawful grammatical errors (this is called "scrubbing"). Of course, you're still left with a shitty base TL, but it's at least better than the HKs-as-is, so these scrubs are always just a stopgap until somebody can do a full TL. And there are some groups who will subtitle a series from scratch just to spite the HKs.
The only thing good about these subs is that they tend to turn into memes. I'd recommend looking up the HKs of Revenge of the Sith, which ended up becoming an impressive stockpile of memes, most famously being the origin of "Do not want!". For memes that came out of shows I actually watch, I'd recommend googling phrases like "Don't molest the lawyer" and "Gao Main Bastard".