Why can't planes take off and trains leave in foggy weather?

So it is not affected by strong winds, heavy rain and fog. This is also the main advantage of rail transit. The plane had to be grounded in fog, strong wind and heavy rain. Because when the plane takes off and lands, due to the bad weather, the line of sight is not clear, which is not only difficult, but also extremely risky. When taking off and landing, once the pilot makes mistakes in judgment and operation, the plane may run off the runway or be destroyed. Therefore, when the vision is affected in foggy days, the airport and expressway have to be closed, which will inevitably cause unnecessary heavy losses.