Keep in mind that voltage dividers have three legs, and the voltage in the junction is determined by all three.
Normally, two of these are resistors with a known value with one side connected to a supply rail (= low resistance), so the combination of voltage and resistance is known and fixed for these. The third leg, the "output", however also affects the voltage, and in a normal voltage divider, you want a high resistance here to minimize the effect.
If you have a high resistance on the output path, it is no longer usable as a supply rail, because that would need to be low resistance — try connecting a voltage divider behind a voltage divider.
A typical buck converter contains a voltage divider on the feedback path to select the output voltage. The feedback input is a high-impedance comparator input, while the output of the buck converter is connected to a large capacitor that acts as a low-impedance source, so the output of the voltage divider is well-defined.