it's about repeatable muscle memory. yes, practice helps. no, not everyone can be a great shooter no matter how much they practice if they can't repeat the motion exactly. some of it is just a physical trait.
Agree with repeatable muscle memory. Add confidence (which is a result of success) and relaxation (easier to relax when you are confident that you will make it) = very good free throw shooter.
When the mindset works on confidence, you most often see free throw shooters focus on precision. Though it sounds reasonable in theory, the over focus on precision (careful AIMING of free throws) usually lowers success. In this case, being carefully over precise works contrary to repeated muscle memory.
As far as Davis & Taylor, as noted by others their mechanics are horrible. Their lack of success creates the mindset / confidence problem, thus their results will never be consistently good. For both Davis & Taylor, it is mechanical and mental. Body language is loud when they approach the line.
Fore's mechanics look less than desireable as well. As a result, Fore's lack of success (and thus mindset) are, I suspect, going south. I think that there is still time to turn him around (sport psychologist maybe, and / or a free throw coach), but if not addressed soon, he could end up like Davis & Taylor.
T.J. Cline seems to me to be rushing his free throws. For some unknown reason, he seems to be uncomfortable. What I have seen is that often when he is handed the ball, he fires a flat trajectory shot up very quickly. I think that if he just relaxs a bit, and
focuses on his mechanics his numbers will go up.