You want to estimate what percentage of people in your town

You want to estimate what percentage of people in your town own a bmw, you call 100 people at random and ask whether they own a bmw if they answer truthfully, why will you still not know what percentage owns a bmw

Solution

This is because of random sampling. We could, by chance, select a sample that shows a completely different proportion than the population. We can construct CI for true population proportion, but we may never know its value (unless we survey entire population)