Bursty current spikes, short and fat traces, using the motherboard as a heat sink, active cooling, and allowing the temperature to rise quite a bit. If you look at thermal camera videos[0], it pretty clear where all the heat is going (although a significant part of that is coming from the voltage regulators).
On the other hand, your national electrical code is going to assume you're running that 350A cable at peak capacity 24/7, right next to other similarly-loaded cables, stuffed in an isolated wall, for very long runs - and it still has to remain at acceptable temperatures during a hot summer day.
On the other hand, your national electrical code is going to assume you're running that 350A cable at peak capacity 24/7, right next to other similarly-loaded cables, stuffed in an isolated wall, for very long runs - and it still has to remain at acceptable temperatures during a hot summer day.
[0]: https://www.youtube.com/watch?v=YyDMlXEZqb0