No, you’re right! But it’s only possible for epub/mobi and txt files.
Pdfs can be zoomed into, but then you need to scroll around the page and it starts feeling rather like you’re looking at the paper through a keyhole. I tried converting pdf to epub, but the formatting/page layout of the original matters and it doesn’t translate well.
I was wondering the exact same thing a few days ago and spent some amount of time googling it.
There’s already such a project actually, called Swanye. There seems to be a single person working on it, so if anyone has time to help I’m sure it will be appreciated