Not even "difficult", just any game which doesn't hand you your victory for merely showing up.

Dark Souls is by no means "easy" but it is a game anyone can beat if they end up wanting to. It is more about patience than it is about raw skill (I beat Dark Souls and Bloodborne many times, but don't ask me to play Contra or to get good at Counter-Strike. That's two leagues above me).
But the bottom line is that it is the most well-known commercial success for a challenging game as of late, so it is a convenient etiquette for game journalists to use.