level shifter alternatives using passives
"Why can the same be not done using a voltage dividor or..."

Because a voltage divider consisting of 2 resistors:


can only interface between a higher voltage to a lower voltage, for example: 3.3 V to 1.8 V
the divider forms a resistive load, when a signal is 1 a current flows. This is unacceptable for low power applications.
it only works properly at the input/output voltage ratio it is designed for
so a divider for 3.3 V to 1.8 V might not work for 3.3 V to 2.2 V
The impedance level of the output signal increases because it comes through a resistor, not from a low impedance output. This is problematic for high frequency signals.


Proper levelshifters don't suffer from these disadvantages.

So in general: no, a resistive divider cannot replace a proper levelshifter.
Maybe in some applications it can but be prepared for all the disadvantages listed above. Is is worth that ? I would say no and just use a levelshifter.