You could convert, but you'd have the converter loses too as bob1029 pointed out. A switched DC going into a transformer can act as a convertor, then a step down transformer, then... yeah. A buck regulator would be better.

If you have something to run that needs constant motion besides the current, a motor-generator might be useful. That can convert pretty directly. I'm thinking like the outboard drive shaft on older midsize tractors.

Switching DC through a transformer is just what you do in a flyback converter, so the two windings of the transformer act like separate inductors to the circuit and the turn ratio sets the duty cycle instead of the output voltage. Tends to be more lossy than a buck or SEPIC converter (though less lossy than a low-frequency AC transformer), but provides isolation easily.

And a motor-generator is far lossier than a flyback converter, they're mostly good for high-power applications where you need to convert from single phase to 3-phase and need isolation at low cost. Even then a VFD (variable frequency flyback converter or inverter) will be more efficient, just more expensive. Though days VFDs are getting quite cheap for their power.

Have you found a buck convertor that takes 400v DC input that isn't actually paired to a transformer for isoolation?

They're almost certainly all paired with a transformer for isolation (unless maybe the whole thing is in a sealed module, but that would be rare). But it won't be a low-frequency high-loss AC transformer, it will be driven at a much higher switching frequency to minimize losses. That will also mean the core material will be different, though as I'm not a power supply designer I can't tell you the most appropriate formulation for such applications. Probably not silicon steel used for 50/60Hz AC transformers though, likely some sort of ferrite.

Basically not all transformers are equal. You need to get the properties right to match the application. In the case of their use as a component in an isolating DC-DC converter (or inverter, or other similar system) they're typically not mentioned separately, since the converter is usually a module.

Well, yeah, but a high-frequency transformer can still itself have a coil differential for step-down and is still a transformer. They make them that operate at tens of kilohertz just fine, which is about where the inverter is going to operate.