That's one complaint I have about it too. But remember: the reward mechanism exists to encourage a certain style of gameplay. If you want players to solve problems diplomatically, give no XP for killing and lots of XP for convincing.
A missing component of the above systems is treasure XP. Typically, early D&D gave lots of XP for treasure and not much for killing monsters (especially at high level). This has gradually faded to the point some editions give no treasure XP. Naturally this has shifted the focus of the gameplay from avoiding monsters and stealing their loot to killing everything regardless of loot. Character endgame rules changed the focus too. Earlier characters were treasure-hungry adventurers saving up for a stronghold, while later ones were bloodthirsty murderhobos. Now we have such a variety, and you can find well-tested houserules all over, so you can encourage whatever gameplay style you want.
The XP for HP system encourages getting up in there and bashing heads. If you also get XP for treasure, there's an interesting choice: is this fight something we need to do, or would it be better to skip it?
There's no XP award for staying at the inn singing songs and hustling hobbits. If your clever scheme converts the Orcs to your side and you get the treasure with no fighting, you still get treasure XP but not XP for the Orcs. If you want to encourage sneaky schemes that bypass danger, that's cool, but you need to somehow incentivise the scheme.
The problem I find with ad hoc XP awards (such as for a good plan or good roleplaying) is that the referee seems like a nobleman handing out trinkets to courtiers. It's a lot tougher to decide how to handle it. And be careful lest your players end up always doing the "sneak, parley, fight, fake surrender, fight more" method.