"Why do Americans use...?"
Big question! We'd have to ask them!
Anyone asking "Is he dead yet?" may show impatience and perhaps feel a bit angered because "he didn't die yet." In the worst(-)case scenario, you or anyone else wants him dead, yet you or anyone else watches the near cadaver still breathing and revolving his eyes. Therefore, you or anyone else says, "Ohmigosh, he should have died half an hour ago, yet he still breathes and revolves his eyes! Let's choke him, shoot him a couple more times, slit his throat, stab him harder this time! This way he'll die quickly!"
To make a long story short, "Is he dead yet?" = "Ele ainda não morreu?"