D&D assumes monsters drop to 0 HP and are dead or in an incapacitated dying state.Why you need to hit the giant’s head?
This requires warriors to be able to strike monsters in their vital organs which cause quick death.
If warriors are simply stabbing at a giant's legs and causing them to fall down at zero the warrior still would be able to fight while sitting on the floor.
"The only hit point that matters is the last one" in every telling of hit point abstraction requires clean blows to vital organs.
So a medium size warrior would have to be able to reach an additional 5 ft cleanly in combat to wound the vital organs an armored giant or dragon or demon or devil.
Their heart is 12 feet up. Their brain is 18 feet up.
And that's before you get into special attacks that temporarily stun or blind.