Some of it is definitely volume, but a part of that is the way that loud sounds bounce off objects, multiple times, and are still loud enough to be properly heard from where they were reflected from and / or refracted around. This sense of envelopment from sound bounced all around you is possible in current games and is massively underrated in terms of its immersive power.
See binaural / ambisonic recordings - e.g.
this, which is giving me flashbacks to my synthesising efforts a few years ago! This stuff is getting some serious attention now that interactive VR visuals are practical, and despite initial skepticism that accurate simulations of sound fields actually make any difference in that application.
As regards the volume issue itself, a bit of psychoacoustic knowhow, some dynamic range compression and half-decent hardware should be all you need. Throw in a tactile transducer with its own dedicated channel and you get closer still. All that's really left is the pain, i.e. the volume knob.
Regarding the accuracy of GT's sounds, there are still missing sources.
I disagree about the Subaru sound, it is mostly exhaust coloured for better representation on the interior. The exhaust is clearly fundamentally the lumpy boxer sound we all expect, the intakes of these modern engines have a dull two-cylinder drone (
example), which is obvious from looking at the manifolds. Missing the intake sound means the exhaust has to be modified to give the same impression.
Also missing in the interior is the way the engine's vibration causes the whole car to vibrate and, coupled with the sound of (the shape and materials of) the space within the cabin, "richen" the sound, especially since the sound comes from all around you. Boxers are not buzzy like inline fours and impart a subtle roughness this way (due to odd-firing cylinders being in two planes, and offset), despite claims of absolute smoothness.
The Renault RS-01 is clearly crying out for intake sound, and again the V6 lumpiness (that firing order...) is not heard in the chassis and bodywork.
I'm not sure if I like the crackles yet, although I recognise they really do richen the soundscape, they do not seem to fit with the rest of the (WIP) aesthetic at this point. At least at the moment it seems as though there's a few different sets of crackle sounds so that not all cars sound the same. I hope they're using AES for it, but the whip-cracks would suggest not.