As you level up, different kinds of defenses become the most important.
At low levels, AC is probably your most worthwhile line of defense, since it can outright prevent the most common effects that might take you out of a fight (i.e. incoming attacks). Good HP are important, but will run out quickly. Good saves aren't that important, since the almighty d20 still ensures very swingy results even if your save bonuses are comparatively high. Should you get your hands on a little bit of damage reduction, that's a good thing, but even at low levels, DR isn't doing much for you.
At mid levels, AC dwindles in effectiveness, but high HP can still keep you going even though you will take some hits, and good saves become extremely important to avoid a sudden end to your contribution in a fight. The amounts of DR available to PCs is completely irrelevant by now. Resistances to energy types can prove randomly useful, but full immunity is probably not on the table yet. Mobility in three dimensions and winning initiative become very, very important assets.
At high levels, high HP are of course still good to have, but HP damage isn't the most dangerous thing you will encounter, and many effects are still devastating even on a successful save (or don't allow a save at all). Your most important defenses are flat immunities to certain groups of status effects (freedom of movement; immunity to death effects, energy drain, mind-affecting etc.).
Good saves can still prevent a bunch of unhappy circumstances, but aren't to be relied upon anymore. AC is by this point near negligible, since most enemies will be able to hit with their most powerful attack forms regardless of even very high AC.
You will often need to know what you're facing beforehand, or expect to die/be made a mind-puppet/be sent to another plane of existence/be heavily resource-drained a lot. Teleport-like mobility and being able to always act in the surprise round are both extremely necessary, and at the very least not losing initiative is really the key to high-level survival.
This phase of the game is often referred to as rocket tag.
There you have it. It's not even a fault in the numbers game per se; it's more the result of what's possible at mid and high levels, it's about what level-appropriate NPCs/monsters can do.