It's even stated within Dota 2's lore that creeps are thralls to the will of the Ancient Primordials and would gladly sacrifice themselves for the betterment of their aligned force.

That explains why they suicidally run down a lane and attack every enemy on sight regardless of how hurt they are. It does not explain why killing your own foot soldiers just to deny an enemy champion the satisfaction makes any sense

I submit to you, once again, that this is a fantasy game where you have talking bears and those that can conjure fireballs from their hand. Denying makes sense both lore-wise and gameplay-wise.

This "It's a fantasy book/movie/game anyway and thus should be immune to any suspension of disbelief problems" is, and always has been, utter bullshit. In any good fantasy setting stuff happens for a reason. And if it is a really good fantasy setting it happens for a consistent reason. A wizard teleporting to the other side of the planet is completely believable. A wizard forgetting that he has this ability at a crucial moment despite not being mentally impaired is not.