A voltage divider is founded upon ohm's law, and assumes that the current going through the resistors is equal. When this is true (ie, negligible current drawn from the middle of the divider), it works just fine. So it's great for reference voltages and stuff. But if you need to power anything with it, you need to use a voltage regulator, which has active circuitry in it that will keep the output voltage constant as the output current changes.