It’s not. That’s why no one is saying that.
They’re saying his inaction and divisiveness have been counterproductive to containing the virus, and more people have died than should have as a result. He bears some responsibility for the numbers in the US, but accepts none.
Xi Jinping and his government also bear some responsibility for them, as it would not have arrived in the US in the first place if they’d acted as they should have.
They are not the only ones who bear responsibility, which is why no one is saying either Trump or Xi killed anyone. The virus killed all of those people, and kills more every day.