You can wash your hands for 20 seconds, scrubbing vigorously, before having any food or touching your face or after coming inside
You could do these things, but would you? Are you sure you always wash your hands for 20 seconds before you touch your face? Do you time your hand washing? Most often the food you eat has been handled by a number of unknown hands. How does washing your hands protect you from an airborne virus like the Coronovirus? Washing one's hands is a good practice. It is not a panacea for every contagion we encounter on a daily, or even hourly basis.
Some people are obsessive germaphobes. They spend much of their lives worrying about catching something. Even if they could create an absolutely sterile environment they'd probably continue to fret about germs and cleanliness. Most people's immune system needs some exposure to germs in order to develop its natural immunity.