Plenty of people use shared computer, especially in environments with low financial resources (ie people in developing economies, low income families in developed countries, etc). This accounts for hundreds of millions of not billions of people in the world
It's unrealistic to expect all these people to have a non-shared computer to use, and unrealistic to imagine the shared computer to be set up by someone tech savvy enough to create separate profiles for people.
If I were to pick a random library or local school in South America or Asia for example, I would bet they have a shared computer where you just sit down at a logged in windows profile