different games have different requirements, you cant just take a racing engine optimized for that purpose and make a halo with it and expect the better results, you will probably get worse results
yes but the car doesnt have to alter its geometry all the time like a humanoid character has to, that is very easy for vertex shaders they mostly pass the data to fragment shaders instead of doing work and adjustments, also there is no joints/skeleton or they are very simple, no ragdolls and the hundreds of animations required for a humanoid characters and its individual parts that affect animations are not required for the car just because something use more geometry doesnt mean its more complex to run, the people and trees are also mostly geometry in racing games with not much else, you can add collision and stuff like that but they are usually very simplistic this situation usually allows to push more geometry also because its optimization is very different, the camera is more fixed/limited and that allows to predict what you dont need to render(on FPS this is more difficult) and geomtry is static so it doesn't require to access ram as much for animation data
for example MS was pushing an optimization for rendering part of the screen in normal(slower) ram they wanted to split render targets to ram and eSRAM to save eSRAM space and get 1080p more easily as you may guess this optimization works very well in racing games because the upper part of the screen has the skybox most of the time, sure you can use it in non racing games but obviously can get more complicated and dynamic resolution may be required when you wont get benefits splitting targets
you can see that racing games that need to run at 30 fps also use more interactive effects and objects, dynamic light and more, you will probably get worse results trying to adapt another game of different genre into a racing game than using it original engine that probably will be already optimized, its the same almost every generation, people see a game that uses too much geometry and think it can be adapted to any game, that is not the case