The point of the rules is to enable resolution of declared actions involving these beings, not to frame an alternative universe in which the same labels are used but the things themselves are radically different.
They aren't
radically different. They're
recognizably different. Beowulf was a
great swimmer, and while someone like Michael Phelps might be a better one in real life, we know for a fact that the laws of physics are at least subtly different because there's no way you could get a huge giant (humanoid), or a gargantuan flier, if they were using our own laws of physics. So they tweak it, to better fit the fantasy stories we want to tell, and the logical extension of those tweaks is to encourage the fighter to go swimming while the rogue sits on the sideline.
Also: what biological property does hit points measure, which a harpy exemplifies to a greater degree than an ogre?
Hit Points are possibly the
only mechanic which are hard to pin down satisfactorily, because they try to make it cover so many different aspects. It
is explicitly the ability of a creature to not be incapacitated by attacks, but that's an aggregate of a number of different features like toughness, skill, determination, luck, divine favor, etc.
Which isn't to say that those factors
couldn't be independently quantified. It's merely that, from our perspective, separating them out is more trouble than it's worth, so we don't have all of the information necessary to accurately reconstruct those factors. The game rules are only a pale imitation of the reality of the game world.
Kind of like how you might have three different components to your SAT score, but the first pass at the admissions office can get enough information out of just your total that it doesn't need to look at the components.