Theory student asks: wait, aren't non-diatonic progressions super common in guitar-based music? Yes indeed they are! Diatonicism is easy and natural on keyboard instruments and in notation, but the guitar's affordances are different.

The usual entry point for guitarists is the "standard fifteen" chords: A, Am, A7, B7, C, C7, D, Dm, D7, E, Em, E7, F, G, and G7. Most untrained guitarists write songs by combining these fifteen chords.

You can make most of the key of C with the standard fifteen, but the voice leading is so horrendous that barely feels worth it. You're just as likely to try combining the chords together in ways that don't fall into any of the major (or minor) keys.

Every rock guitarist tries pairing E major with G major. The blues emerges effortlessly and naturally from this kind of experimentation. Also, pentatonics are vastly easier to play than full scales, that helps too.

If you learned music theory "the right way," you have no idea how weird it is to learn it via rock/blues guitar. A whole different universe. Diatonicism feels positively exotic.

"Good" voice leading on guitar requires all kinds of advanced technique. In rock and blues world, it's all just parallel fourths and fifths and big jumps. It's exciting to discover F to D/F# to G, but that stuff is rare at first.

Meanwhile, you usually discover bending and microtones almost immediately, and it's super critical to having a good lead sound, and no one EVER talks about that in theory contexts.

So you start thinking, either, I guess my ears are wrong that all these microtones are important, or theory is not helpful in getting me where I'm trying to go.