I'm just going to talk about 2014 here because it was the year I really got back into following monster trucks and it was the first year the backflip ramp was regularly used in competition outside of Vegas.
The cars weren't really on the track in 2014 (and a few years prior), they were in it. They were just there, not getting crushed at all. Here's two pictures from Foxboro 2014. Dragon came out in the middle of racing, Northern Nightmare towards the end of freestyle. The cars built into the track are in almost the same condition in each shot.
The past couple years there's still been plenty of crush cars out on the track. (Though they did take some off turning cars the track at Foxboro this year and not use them for freestyle.)