Eyesight changes over time. People learn new languages, and want to switch. Making these decisions for the user and then forcing them forever is decidedly un-user-friendly.
Better than cramming every such feature as a first priority, and making a UI that's confusing for everyday use.
They can always look them up (online), set them, and forget them. Better than having all such settings front and center over more commonly used settings.
But they don't. You're a reader of HN. It is fairly obvious that you're the sort of person who, at a point when you need to accomplish something you don't know how to accomplish, will assume that it's possible and start with research. But there are a lot of people out there who will start with the assumption that not being able to see their phone screen so well anymore is just something to put up with. The entire point of this post and discussion is that Apple may not be providing the best experience for those users for whom it would never even cross their mind that changing the text size on their phone is possible.
Even most young people I know are not able to google things (even if it's some important information they urgently need). Or maybe they just don't get the idea of even asking the internet.
It's not like you're learning a new language everyday. Nor losing your eyesight. Keep in mind that the life of a cellphone is anywhere between one and six years, for most tends to be around three (speculation). Isn't reason enough to make it a top priority for people to be able to access language / text size changes in their settings everyday.