I think it's a combination of
(A) 35% Natural Skill
(B) 30% Constant Practice
(C) 20% Equipment
(D) 15% Setup Knowledge
(E) x% Desire to Win (Motivation)
when you look at these guys compared to each other.
(A) Some guys don't need constant practice. Look at Jukka. He works days at Losi, has a family, goes to a big race and can place in the top 5 of any off-road race he goes to. Mark Pavidis is the same way but he's more versatile and and do equally well in just about any class, off-road or on-road.
(B) Some guys practice all the time but don't have the skill or equipment, like the guys at your local track who always seem to be practicing but never seem to do well.
And as for (C), the next time you're at a big race like the RRoC or Cleveland, keep your ears open for the pro guys complaining that they didn't get a good pack of cells, or they chose the wrong insert, and see where they're at the in rankings. In big meetings a difference of 5 seconds on the track over 5 minutes can be like 20 to 40 spots at the end of qualifying.
(D) Knowing how to set up your car helps you be a better racer but it's not necessary. Masami has a pit man - his dad, who works for Yokomo, and so do many kids that are club racing. It definitely helps but it's not a crucial part IMO.
(E) Motivation can be the biggest part of the equation and varies from pro racer to pro racer. As the older pro guys age they see the younger more daring guys coming up the ranks, they look at how much they've accomplished and some of them sit back and rest a bit. Others keep going until their sponsors start drying up and their hand-eye coordination starts getting a little worse.
I read once that Masami practices R/C in some form every day. Think about that. 13 world titles.
Fast-paced video games are a good way to work on your hand-eye coordination (which makes up most of the Natural Skill, IMO). Guys with great hand-eye coordination can be good at r/c racing, skeet shooting, kart racing, whatever.