Cause you can still get killed?
-YRUSirius
I deserved that for asking a rhetorical question.
But I think the point stands. It's not that Dausuul or I are
incapable of providing PCs with an appropriate Wealth By Level allocation. We just don't think it's a good thing to hinge game balance on.
If my players comes up with some great moneymaking idea or lucks out and beats some encounter way above their heads, I want to be able to reward them with a giant sack of cash. If that translates to a giant sack of HP, I'm going to be more stingy, which just sucks for my players.
Similarly, if the game has an option for this extra pool of GP-based HP, encounter pacing will change based on your party's wealth. If your campaign falls under the Wealth By Level, characters don't have staying power and daily effects become more prevalent. If your campaign is over the Wealth By Level, they can afford to push further and daily effects will show up less.
It's trivial to get the wealth dial to the "right place," but there are plenty of reasons you might want to have more or less gold in your game. Pinning something as important as HP to wealth makes the decision "I want the PCs to be able to afford a fortress" suddenly carry the baggage of a "short rests include full healing" houserule.
And, yes, all of that can be fixed with houserules or never deviating from the assumed Wealth By Level. Or they could make healing potions work like other medicine and we'd never have to worry about it.
Cheers!
Kinak