Why not just use a voltage divider? Several reasons: cannot drive variable load, wastes power. [include diagram of resistive divider, 12V split across 2 1k resistors, + simulation results for loads < 1k and >> 1k.]
For converting to very low DC outputs, sometimes a resistive divider suffices! But for anything else, we need higher efficiency, defined as $\eta = P_{out}/P_{in}$.
Thus for a DC→DC converter to be effective, it cannot be “always on”. This leads to a class of devices called “switching converters”. Surprisingly, it is possible to boost voltage with only a single active component (a transistor serving as a switch)! Buck/boost converters take advantage of the energy-storing properties of inductors and capacitors.