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.