ATX power supplies are a much easier task. Fixed voltages for everything. A USB-PD charger capable of 240W is essentially a variable power supply from 5 to 48V. Much more difficult in execution.
Rather the opposite, actually. ATX power supplies have to supply -12V, -5V, 3.3V, 5V and 12V at the same time. A USB-PD charger only has to supply a single voltage.
And pretty much all power supplies are switched-mode power supplies anyways, whose controller usually requires you to explicitly define the output voltage using feedback components. Changing that to a variable power supply like USB PD doesn't really require much engineering.