Flight instructor pay is the problem. Pay is linked to income generated for the operator, so instructors will always be at the bottom of the food chain. So, on the way up most do it do build hours for a better future. A few within those ranks are dedicated and do a great job, for as long as they are there. Sadly for students and flight schools, these are usually the ones that the airlines will eventually take.
This is NOT saying that there are not some good career instructors. I was very privileged to have been mentored in my early days by just such a guy, now passed on to the big hangar in the sky.
But is a good pilot a good instructor? Not always. Sometimes a pilot is so good he /she just does it naturally and so has a hard time understanding how anyone else could find learning to fly or upgrading to new levels in flying a difficult task for many lesser mortals. But if an instructor can't demonstrate mastery of whatever is being taught, that's no good either.