Why Planes Can't Land in Fog

As we know, driving in fog can be a terrifying experience, but what about landing a plane in foggy weather?! Back when airplanes first made an appearance, they could only fly during the day, and only if the weather was good. Pilots had to rely on their own eyes to bring the aircraft down safely. What about now?
When planes are ready to land, they first check the forecast. Pilots dial a specific frequency in order to listen to the current weather conditions. If there are patches of clouds when aircraft are about to land, it’s safe. But, if there are thick clouds that are too close to the earth, then there’s a slight problem. So how do airplanes land with no visibility, while hundreds of flights get canceled for the same reason? It’s time to clear the air about this mystery.
Other videos you might like:
Why Planes Don't Fly Straight &
Why Planes Don't Fly Over Antarctica &
A Plane Disappeared And Landed 37 Years Later &
TIMESTAMPS:
How to measure the height of the cloud ☁️ 0:46
The Instrument Landing System 3:09
How does fog cause delays when there are ways to work around it?