People can think like this because the vast majority of people, Protestants especially but also most Catholics, simply don’t understand the Catholic Church and what it
really teaches. Also, there is a very strong anti-Catholic attitude out there. When I was a Protestant, I did it too. Because of the way Protestants are taught, they often take issue with the most minor of things (like this). When it comes to important things, like Mary, they will say the beliefs are wrong, take away from Christ, and say other things that pertain to salvation and important matters. When it comes to less important things, like what a Saint is, or something about nuns, they will just ridicule the beliefs. In other words, they’re not important enough to condemn, but they are representative of the opposed (at best) or despised (at worst) Catholic Church to make fun of and deride as gibberish.
As far as this specific teaching is concerned, you may want to explain to your friend that the Catholic Church believes that all Christians are saints too. (and this is a very simple explanation but it’s good enough to tell to a Protestant

The Church simply keeps a list of the people who lived exceptionally Christian lives that we can be inspired by. They are called canonized saints, but usually people drop the word canonized and just say the saints.