Why are people racing the same combos every night? Because they are human. They want to win, not tomorrow, but right now. They don't want to spend time finding unpopulated servers with fresh new combos and look for a suitable setup. They want to sit down, start up the game, race with lots of other players and finally win.
If you want some organized racing with a large variety of combos, join a league - cliché, I know, but that's reality. It's getting impossible to just fire up LFS and get a race as furious and various as a few years before. Here I'd like to mention Counter Strike:Source, because a same kind of behavior can be seen there, too. A lot of servers are running de_dust2 only, and the most of them are quite densely populated. I myself am finding it hard to play some other maps (ie. not office/italy/aztec/dust) at least 5on5. Players just want the same old sh!t which they know so they are able to pwn with the obvious desert Eagle + awp/ak/m4a1 combination, which again leads me to the fact that most of the players want to jump straight to the part where they are winning, but not learning anything more.
Back to LFS and the problem of some unrealistic racing on kerbs 'n stuff - why use something different, when there's a well tested and working receipt for quick winning? Players will exploit the flaws of any game if they can get an advantage doing so. RL drivers won't go over some of the kerbs in LFS so drasticaly because of the damage that may cause to their racing car, but in LFS, nothing stops you from using anything on the track to your advantage. In LFS it's simply way too easy to drive constantly on the limit (and I'm saying this not ever setting a WR and not even being the fastest driver out there), pushing your car extremely because there isn't that "fear factor" as in RL.
Boris, you said that at least the community of iRacing is 18+. I don't think the age of the player is important, it plays a role, but surely anybody could find an example that would contradict with the statement "the older he is the smarter he gets". And I don't understand why are some of you "scared" of the fact that others are playing/trying out other sim's out there. By buying LFS you didn't bind yourself not to play anything else o_O . But on the other hand by buying LFS you agreed to the way ScaViEr are developing it. It has been said a thousand times already that the devs aren't obliged to listen and respond to the alltime whining about the flaws and deficiency of the games present state. It's still in Aplha stage, so I don't really see a problem with the game right now. It's not final, most companies wouldn't let anybody else other than betatesters to play a game's alpha version >:| Waiting can be hard, but you don't have to wait right "here" and comlpain every day about how long does it take to get the frickin patch out. The ending is a bit offtopic, but I hope some will learn from it, or at least think about it.