...said no one ever.
But it is. The entire story is based on scientific speculation. What if the cordyceps fungus mutated and started infecting people? What if there was one immune girl and what if the only way to produce a vaccine was to kill her? This kind of questions are not common in your typical zombie apocalypse fiction. Even the sequel abandons any scientific concept, for that matter. I Am Legend, the book that started the modern zombie apocalypse subgenre, had a protagonist who was trying to find the cause for the outbreak. Most of these stories, though, simply say it's "some virus" that caused it, if they mention anything at all. Therefore, TLoU is a great sci-fi story.
Also character models in the game remain top notch for me