The traces are extremely short. Look at a modern motherboard and you'll find a bank of capacitors and regulators about 2cm away from the CPU socket.
If you've got 4 layers of 2oz copper, and you make the positive and negative traces 10mm wide, you'll only be dissipating 28 watts when the CPU is dissipating 300 watts. And most motherboards have more than 4 layers and have space for more than 10mm of power trace width. And there's a bunch of forced air cooling, due to that 300 watts of heat the CPU is producing.
Electrical code doesn't let buildings use cables that dissipate 28 watts for 2cm of distance because it would be extremely problematic if your 3m long EV charge cable dissipated 4200 watts.
If you've got 4 layers of 2oz copper, and you make the positive and negative traces 10mm wide, you'll only be dissipating 28 watts when the CPU is dissipating 300 watts. And most motherboards have more than 4 layers and have space for more than 10mm of power trace width. And there's a bunch of forced air cooling, due to that 300 watts of heat the CPU is producing.
Electrical code doesn't let buildings use cables that dissipate 28 watts for 2cm of distance because it would be extremely problematic if your 3m long EV charge cable dissipated 4200 watts.