I sang off key for years, then found an exercise that worked (humming scales while away from the instrument), and a couple months later I found myself being ver

I sang off key for years, then found an exercise that worked (humming scales while away from the instrument), and a couple months later I found myself being very on key, harmonizing on the fly, noticing tiny off-key moments in popular songs, and getting compliments about "talent". So I think a lot of these skills are just taught wrong. You can teach an untalented person to "get it" - Betty Edwards' books famously do that for drawing skill - but for many skills the right method of teaching hasn't been found yet. It's easier to pick & choose students who already "got it" and then take credit for their success (like most music education, and higher education in general) than to research teaching methods that will work on more people.