Why aren't non-isolated DC-DC converters made for high wattage applications?
Like Aaron said, a topology with a transformer allows for better control of the step down/step up voltage. A duty cycle of 7% seems fine on paper, but in reality you should aim for closer to 30%. In an application with changing load that 7% nominal duty cycle can drop down to 1 or 2% with low or no load. 

When using a PFC, especially a boost PFC, the 400V(ish) output is chosen for the AC line. A universal input converter (90VAC-264VAC) will have a rectified voltage of 128VDC to 374VDC. If you are only doing low line (90-130VAC or so) then it doesn't need to boost to 400.

Then there is safety concerns. Any company is going to have to get UL/CE/TUV/KC or whatever other safety agency you can think of. Think about a buck converter, if that top switch fails, how are you going to protect the output from that HVDC? A transformer/coupled inductor provides galvanic isolation. 

Then you start getting into efficiency concerns. A buck converter will always have turn-on, turn-off and conduction losses in the switches. There is nothing you can do about it. A resonant topology like a PSFB or LLC can lessen or eliminate the turn-on losses. Look up Zero Voltage Switching. Phase Shifted Full Bridges are really cool and a topology I work with often. 

You can use a buck converter for a 1kW source, there are just better options. Of course, better is the enemy of good! For a medium power source like that, you can look into interleaved buck converters. These are essentially 2 or more buck converters connected in parallel.