I have heard both Merle and Willie say (in TV interviews on the country channel), that they both play/played open chords mostly, and don't
read music.

On the same channel on the life of Hank Williams, it was said he too played basic open chords for his songs. They picked up their skills mostly from other people who played guitar (like most people imho.)

Nothing wrong with learning theory if you feel you need it though.

Tom in S. Dakota