Both zener diodes and voltage regulator ICs have the same purpose: to keep the output voltage constant when the input voltage varies.
A zener diode voltage drop varies significantly with
current, temperature and also does vary with the load.
An regulator IC has 4 stages:
1. a voltage reference (thermocompensated or not)
2. an error amplifier (who amplify the reference voltage)
3. a series pass transistor (who drives the load)
4. a feedback circuit
The voltage reference, amplified by the error amplifier, drives the series pass transistor, who drives the load.
When the output voltage drops or increase in changes with load, the feedback circuit senses the change, and "send" that change to the error amplifier, who inverses that voltage and drives the series pass element in the opossite direction.