This is false equivalence, both from a rules perspective and (presumably) from a character perspective.
From a rules perspective, there are no rules for stabbing anything in a particular anywhere, while there are rules for 'a fall from great height'. Weapon damage tables reflect abstracted effectiveness rather than specific effects. In the case of 'I stab myself in the heart' you aren't 'overruling a rules exploit', you are making a ruling where no rule exists.
From a character perspective, by the time the character has the hp to reliably survive max fall damage, they've probably fallen a few times and survived. They may have seen other heroes survive these falls, possibly other even more impressive ones. In contrast, they have (probably) never seen anyone getting stabbed in the heart and survive.
So in one case you're making a ruling that overrides existing rules and may also run contrary to the character's (and the player's) experience of the world.
In the other, you're looking at making a ruling in the absence of rules, and your ruling reasonably reflects that character's experience of the world.
Fair enough.
What if they haven't fallen or seen anyone else fall any significant distance? I've played in campaigns where we made it to mid/high levels without ever taking falling damage. If the party contains a character who prepares feather fall, this is actually quite likely.
In any case, I disagree that a few falls would justify a character having a clear understanding of hit points and the "rules" of falling. The dice are variable. Sometimes a character might fall 60' and only take 6 damage. Other times they fall a mere 30' but take 18 damage. Even if we accept the rules as "physics", it would take a large number of falls to get a proper picture of how it all works. Sussing out the 20d6 cap would require many falls from heights of 200' or greater, which not many creatures could possibly survive in the first place.
That's neither here nor there though, as I don't ascribe to the idea of rules as physics.
The only circumstances under which I think that actually makes sense is if the premise of your game is that the
players are in an MMORPG or something along those lines, in which case the premise gives you permission to meta game, at least within the context of what the player-characters would know. The players know about hit points because they can see health bars above everyone's heads.
On the other hand, in a standard TTRPG setup, I don't think that characters are meant to be consciously aware of HP or how falling damage functions. A high level character is confident in their skills and knows they're lucky, but has no concept of hit points. Only the player knows about hp. When the character has lots of hp, they feel great, with attacks that should have killed them going wide. When they are low on hp, they feel like they are on the ropes, with lethal blows coming ever closer to landing.
Hence, the character has no idea that they can walk off a 1500' cliff and survive without serious injury. Even if it has happened to them before, they'd simply believe that they were immensely lucky. However, no sane individual would want to push that luck and risk almost certain death just because they're feeling lazy. IMO, the only way to "justify" such action is if the player is meta gaming.
The characters perceive their world in essentially the same way as we perceive our own world. They are therefore unaware of game abstractions such as hit points, which only exist in the context of the game, rather than being a construct within the campaign world itself.
