[foxyv]

Because, it is much easier to buy a cheap 230V AC inverter and plug it into your server power supply than it is to make a bespoke power supply that takes 12V and distributes it to various components. Inverters are stupid cheap right now, and 90%+ efficient.

[electroly]

For folks outside of warzones with plenty of time and access to international shipping (i.e. not OP), there are off-the-shelf adapters that accept DC input and produce the ATX voltage rails with the proper motherboard plug. Search for "DC-ATX" or "PicoPSU", they're pretty neat devices using DC-to-DC converters.