I guess you could chalk it up to a difference of aesthetic opinion. I don't want to think about buffers or readables. I want to think about loops and strings. Looping over a collection means `for range`, so I'm gonna assume that Just Works. Maybe `for range` is just syntactic sugar for `bufio.Scanner` under the hood, but I don't want to care while I'm using it.
I want to think of a file as a black box full of strings. What's actually in the box? Don't care. How do I get lines out of the box? `for _, str := range blackBox`, same way I loop over every collection. How does that actually work? Don't care. Whoever implemented the box has to care, of course, but I sure shouldn't. I've got more important things to worry about, like whatever it is I actually want the code to do. Every character that isn't about whatever it is I actually want the code to do is a problem.
Having primitives and builtins that only work sometimes (specifically, with a short list of builtin types and aliases for same) means I can't just use the builtins without thinking about what I'm using them on. Having to crush down to a lowest common denominator means that what's in my brain while I'm reading and writing code isn't strings and what I actually aim to do with them, it's how the Reader API works, and whether I read a full or a partial line, and whether I need to handle errors before or in or after the loop this time. I want to think about my problem domain, but Go keeps dragging me down into the weeds.