Imagine your microwave goes funny and blows a fuse. You're upset but it's a minor issue because the fuse is either in the socket or in the plug, or in the fuse board. The rest of the circuit or circuits continue to work. The lights still work. The other sockets still work. The kettle works.

Now imagine they're all on one circuit with one fuse. If that first blows because you were a bit too ambitious with the microwave porridge, all the electrics will fail until you swap the fuse.

A fuse is one of the simpler things to fix, too. With code as well as other fail-points in the system, it's only a matter of time before the whole thing breaks and becomes unfixable because (i) the fault cannot be found (ii) the fault can be found but the part can't be replaced (iii) the fault is a minor issue but it's prohibitively expensive to fix due to its place in the system but it can't just be removed because it's been designed as a critical element in the system, etc.

Reducing the cabling in a car means putting more things on fewer circuits, leading to the issues above. Problems become harder to diagnose and fix. Additional things are like if in an old car if the radiator breaks, you might be able to drive it slowly to a garage but if there's a sensor that won't let you drive with a faulty rad, you aren't moving and if that sensor is on a circuit that can't just be disconnected because it's also tied in to the ignition, you're still not moving. Multiple circuits let's you minimise the kinds of faults that can bring the whole thing to a standstill.