Actually, the car in LFS jerks you around quite strongly, it's just not a very visual effect. If you look at the G-meter (F9 menu) you'll see that for example in the XRT, when power shifting from 2nd to 3rd, you'll go for example from an acceleration of 0.33G down to -0.02G as you shift, then jerk up to 0.47G when you release the clutch until the revs match and you continue at 0.32G.
All this would be felt as a quite violent jerk in real life, but the car doesn't dive much or anything. When testing, I just noticed that the "1g head tilt" effect only works on vertical G load, so acceleration or deceleration won't tilt your head at all, which probably contributes to the illusion that the shifts in LFS are ultra smooth.