Winning races is hard. And since advancement in the game pretty much depends on you winning races, advancing is really hard. See those shiny awesome trucks that could make short work of Everest or Kilimanjaro? You might actually climb either of those peaks before you'll get a chance at driving them in career mode. You can play around with them in arcade mode so you can see what it is you're missing, though, so you've got that going for you. Which is nice.