If I can over-simplify a bit...
The main problem is that your load (whatever you're trying to power) is in parallel with R2.
When you put two (or more) resistors/resistances in parallel you have two paths for the current and the total resistance is reduced.* That changes your resistance ratio... More voltage across R1 and less voltage across R2 and your load.
Voltage dividers are used for low-power signals where the load resistance is very-high so it doesn't screw-up the voltage.
With low-power signals, you can use relatively low resistance values in your voltage divider. But in a power application, the voltage divider resistance would be too low and it would take more current & wattage than your load, so it's very inefficient and it's just not done.
If the load is constant (say a regular-old incandescent light bulb) it's possible to use one resistor (R1) and the light bulb instead of R2. That's very rare...
We do use a resistor in series with LEDs to divide the voltage, but LEDs are diodes and they are non-liner (their resistance changes with voltage). It's still a kind of voltage divider but the calculations (and the behavior) is different.
- There is a formula for parallel resistors, but it's easy to remember that two equal resistors in parallel are half the resistance. If you connect two 8-Ohm speakers in parallel you have 4-Ohms.