Again, there's a huge difference between being a hard worker and being willing to work hard at various aspects of your game. Just because you can put in 2 hours every day at the batting cage doesn't necessarily mean you're equally willing to take grounders till your palm hurts. I have no doubt that Tebow, RG3 et al work really, really hard. I question whether they work really, really hard at the things they actually need to get better at.
I killed myself training when I fought. Four to five hours a day, six days a week. There was nobody who could say I wasn't a hard worker. But, I hated working defense. I was a come forward fighter, and could never be bothered to work on my defensive game. Was I a hard worker? Yes, but I was also stubborn and lazy.
Add to that the fact that it's incredibly difficult to be willing to take a step back to take two forward. These are people who've dominated at every level since they were 5 years old doing what comes natural to them. Now they have to go against reflex and muscle memory and spend a month, a season whatever sucking. It's hard. It's frustrating. If you're a pitcher who could throw 90 since high school, but with a motion that's going to get you Prior'ed, it's a ton to ask to spend 3 months sitting in the low 80s while you learn a new technique, rewire your muscle memory, build up different muscles, etc.
I'm a pretty good powerlifter. Not the RG3 of powerlifting, but I've won a bit here and there, set some local records here and there. I'd be a lot better if I took 100 pounds off the bar and learned proper technique. Rather, instituted proper technique. I know how to lift, I can tell other people how to lift, I know what I should be doing. But, it's just insanely difficult for me to start over with the weights I was using in high school in order to learn to use my body's leverages better and eventually hit new PRs. Again, I'm stubborn and lazy.
It's impossible to master technique without a certain level of athleticism. I've boxed with guys for years who could never look right throwing a punch. You need a baseline amount of coordination. RG3 has that athleticism. You can say that for the same reason you can say that about Rodgers - his fluidity of movement, his reflexes, his bodily control, etc. He has the athleticism to make his body do and go what and where he wants. If he's been unable to nail down proper footwork given that level of athleticism and coordination, there can be only the excuses I mentioned above.
Even the simplest tasks involving sports require massive amounts of technique. Look at running - something we've all done since we were 2, 3 years old. Go to Charlie Francis's site sometime and read how to run properly. Stride length, hand configuration, how to swing your arms, how to position your mouth, breathing, how to relax your traps, etc. Lots of guys are naturally fast and never bother to start from scratch and learn to run to their full potential. Lorenzo Cain did, and look how it paid off. Gronk could probably add a MPH or two to his topline speed if he really wanted to learn to run right. But, again, after 20-plus years being the biggest, baddest and the fastest, who wants to add time to film study, weight training, regular practice, etc. learning how to do something all over again?