Why does voltage change with load in buck/boost converters?
the duty cycle and input voltage determine the output voltage


This is true ONLY if the converters are operating in continuous conduction mode.

As for why a feedback mechanism is necessary &mdash; most of the time, the input voltage is unregulated, and the goal is to have a regulated output voltage.

Indeed, in applications where the input voltage has sufficient regulation, no feedback is used. You'll frequently see this in point-of-load converters, for example. There are several manufacturers of fixed-ratio converter modules for exactly that purpose.