The easy answer to this is that it's quite easy to mass-produce the IS/ID/RB donkeys - get 18 year olds, play them exclusively at C, train those three skills, watch their salary explode! Same thing with PGs. But training a good SF takes significantly longer and you've got to play them at different positions, which is why there are much fewer of them, which more than justifies not only a salary premium but the TL premium.
If you remove any sort of wage penalty for overtraining players, everyone will just have players trained up near or past 20 in whatever skills they want.