Yeah, I know what you mean. It seems like car ESCs are rated the same way aircraft ESCs are, but airplanes pull a much more constant amperate, but the average is higher than car use.
Personally, even though the FETs are rated conservatively and derated properly for temperature and the size of the heatsink, I wouldn't run anything near its amp spec because cars are very peaky and bursts will hit much higher. Burst ratings are much more applicable rating IMO. Plus, there are FET switching losses (the square waves are not perfect square waves) and losses due to the rdson value, both of which cause too much heat in an area too small to be cooled effectively.
So, I too tend to look for ESCs with a continuous rating close to what I think my burst will be.
Hopefully soon, there will be a guide to help people select the right system.