5400rpm HDD´s are commonly used in laptops because generates less temperature.
And? Laptops are no PCs? Therefore Desktops are no PCs either, right?
Also, most of modern eco drives work at about 5400rpm. And I wouldn't call it unusual to find 5400rpm drives in average PC builds. Calling 5400rpm HDDs irregular is not right.
But of course, 7200rpm HDDs are pretty much standard for desktops which are used for gaming, unless the use of SSD is preferred.
Both games works in sandbox environments but both uses different techniques and way different amount of post processing and other level of detail.
Also structures in a sandbox environment "eats" a lot of resources as well, as shadows, reflections, lights/lighting, you need to put cubemaps everywhere for every little details etc...in a racing game that is not needed.
Just because it's usual, doesn't mean it couldn't be done different. What happens if you add rubber which detaches from tyres, moveable murbles, leaves, wind, living environment, detailed car damages with parts and particles flying all over the place, shadows, waving grass/trees, moving 3d spectators, reflections on car/rims, animated animals, visualized hot air effects, loose gravel/dirt/dust, animated drivers, glowing brakes, water reflection, animated water, flies, exhaust flames, aa/af, etc SHALL I LIST MORE? I can, no problem.
Why is it
needed in a shooter? It's nice to have, but needed? A counterstrike as very popular PC shooter doesn't need that many effects, but it's still highly esteemed by lots of people and a big number in e-sports.
Elementary question: Does a racing game or shooter need to look good before it can possibly have a good gameplay?
Is not the same at all. Shooters are more demanding than racing games overall speaking.
The developers define how demanding a game will be, not the game or genre itself.