Mostly they can spread it to you and to any other animals around and by the time you’re aware that they’re infected, they’ll be seriously ill. Or shorter; the chances x benefit of them they developing immunity are much lower than the chances x cost of them becoming ill and having the virus mutate in a much worse direction.

Watching your chickens die sucks, my friend. They don't just drop dead overnight. It's often many days of watching them slowly suffer.

Then imagine spreading that misery to all the wild birds you love in your neighbourhood.

Ethical reasons should prevent you from intentionally infecting them.