...a dagger deals 1d4 damage, and these types of players rarely extend the logic to say "I stabbed that 13th level fighter with a knife, why isn't he dead?".
This is a common and basic misunderstanding of how HPs are/were intended to represent the combat stamina of a creature.
Originally, they were a design compromise by Gygax & Arneson to measure the combat stamina of a creature within a fight, borrowing heavily from their joint background in tabletop miniatures wargaming. Not only do HPs measure a creatures ability to take physical punishment (wounds) but also their ability to keep moving/dodging/weaving/parrying/mental will/morale fortitude (combat stamina - ability to stay in a fight). Thus, it is a conglomeration of homogeneous fighting concepts represented by a single number.
HP is a single number which unfortunately does not take into account the slow degrading of a creatures combat stamina over time - a creature can fight at 100% optimal fighting ability from 100 HPs to 1 HP before becoming suddenly combat ineffective (dead? unconscious? exhausted?). I hope this explanation helps better understand that HPs in D&D are not just "the sum of all wounds".
