If it wasn’t the default, people with poor vision would most likely wouldn’t even know how to activate it by themselves.
Most regular users, especially older ones that are more prone to these conditions, aren’t technically savvy enough to rummage through app settings to learn how to toggle accessibility features.
They’d just try to put on reading glasses and then struggle to use the app as it is.
I think it makes more sense to have sensible defaults like that be activated and make it opt-out for those that don’t like it