I learned this skill on Rfactor, started using driving aids with the Rtrainer car, then moved to formula 3, and then formula ISI (F1). Each time turning off the aids for some practice, but putting them back on so i didnt get too frustrated in the races. Then i moved to playing Fsone2009 and reduced the aids more, again trying some practice without any.
Then went back to Rtrainer, no aids at all, worked up to F3 (a big jump) and learned to drive those without any aids, and it wasnt easy, before moving to Formula ISI again (Driving the Bmw Sauber 2007 F1 car) again with no aids. Then moved back to FsOne 2009 with no aids, along with Series Internationale Du Mans, and a few others that i was previously unable to drive without aids. Later i got Ferrari Virtual academy, which while it was very hard on brakes and understeer felt very easy for me to drive fast (i was at massa's time within a few days of on and off playing)
Somehow this transition period was really important, and what seemed like an impossibility at the start had turned into something that wasn't easy, but did feel quite natural, and very doable. Throttle and brake control were the things that i had developed most of all, something that take a lot of precision.
Anyway, come F1 2010 (a game not made for the hardcore sim racer) i was instantly able to drive without any aid's, and to be honest found it quite easy, though i had a lot of learning to do to get dialed into the driving style and the way the game feels, the grip and the brakes, getting up to speed took its time (and i'm still not as fast as some), but i was able to jump in the deep end with expert/legendary modes.
I can understand that a player who hasn't had the experience that some have had, will find it hard to understand how others can drive without aids at maximum difficulty, but if you give it time, patience and you persist, pretty much anyone can get there and it will feel natural. Games/Sims are completely different to real life driving/racing in many aspects, and it can take time to get around that lack of physical sensation.
Edit : To Anthony - I cant show with enough enthusiasm how much enjoyment and speed that driving with a wheel brought to my gameplay, i played gran turismo 4 (and a little GTR2) for years with a gamepad, and while it was awesome and i became quite proficient at it, the experience and the speed at which you can develop driving skill with a wheel is quite overwhelming. I recently broke my wheel and i feel lost without it while waiting for a new one, i haven't been able to play any racing games because of it, i cant go back to a gamepad now.