Humans and cats have different biochemistry, and whatever it does to get cats to expel the virus from their lungs needs to be chemically a bit different to get humans to do the same.

Because there’s a whole range of processes involved in whether or not a pathogen can infect an organism (mode of transmission, various barriers including immune response, etc.) and a whole different range that determines whether or not an infected organism can spread a pathogen (mode of transmission again, viral load, vector capacity and competence, etc). For instance: assume the pathogen can infect an organism but can’t replicate often enough to reach the required viral load in the host to spread further via it’s usual mode of transmission. We’d end up with a dead end host instead of a pathogen reservoir/vector and the chain of infection would stop right there. That can be seen in the West Nile Virus for example when it infects humans or horses instead of it’s normal host: birds. I don’t know enough about influenza to tell whether that’s a thing here or not but I hope it clarifies why infections are often not that straight forward especially if they involve zoonotic events.