Why are dead things in my chimney and why can't I burn them up?

So was talking to someone recently about fireplaces who said since we haven’t used our fireplace in years the chimney would have to be cleaned first to get rid of dead birds and stuff. Why are dead birds likely in my chimney in the first place? And why can’t I just turn on the fire and incinerate them? Why can’t I smell their rotting bodies?

(Ever since I was told this everytime I pass my fireplace I think of dead birds in it and it’s freaking me out).