Planets of Hats gains its name from the TV Tropes entry Planet of Hats. In which, the unrealistic expectation of entire planets dedicated to ONE THING exist.

Realistically speaking, you can not have an entire planet of dancers, painters, gangsters, mechanics...

Somebody has to flip the burgers, manage the infrastructure, grow the food... and so on. A true Planet of Hats cannot logistically exist. And C.M. Weller used the Hevun's Child trilogy to highlight and lampshade this.