Why do these people exist in times of pandemics?

There are people in society who are going to believe whatever they want to believe no matter what you say to them. Sometimes the harder you try to convince them, the more they refuse to accept reality.

Dealing with campaigns of misinformation and disinformation also makes it difficult for people to know what is real anymore.
A lot of people are easily influenced into believing things that are not true, depending on the sources they trust. That's why compromising mainstream media ( tv, newspaper, radio ) is so effective.