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.
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.
It's significantly more difficult to get a working DC-DC setup up and running that's actually reliable and safe. The power supplies on these devices expect 220V AC so it's easier to just feed that in, and rely on the onboard power supplies to handle the complexities of creating a DC source that is within tolerance for the actual device.
But the server and phones use 12V/5V DC. Why the extra step of DC 12V (inverter) -> AC 220V (server PSU) -> DC 12/5V ?