As for how XP are gained, I think the US has a different approach from Europe. In the US, D&D evolved from wargaming roots and always relied on combat as an important gameplay factor. Which is why many games have very elaborate combat rules that can cross over into wargaming or even boardgame territory. The European approach feels more story-driven and character based, with combat as an adjunct instead of a core component. But the European RPG industry was of course still heavily influenced by the US approach.
I don't know if it's US vs Europe. It's mostly wargame-derived vs storytelling-derived. For instance, Call of Cthulhu had rules for combat (basically the same rules as Runequest because the basic system is the same), but you weren't expected to fight much because combat with guns and without armour is very, very lethal, and combat against Cthuloid Abominations From Outer Space is even more so. Whereas D&D, Traveller, Space Opera (and even RQ to an extend) was based on medieval western fantasy: frontier mentality where guns are always Plan B and are often Plan A, and there are wolves and aliens (or indians). It's more "frontier RPG" vs "urban RPG".
Europe doubled down on this because 'frontier' has not existed in Europe for quite a bit of time and has not been glorified since the decolonisation - but delve a bit into colonial-period RPGs produced in Europe and you will see the same kind of heroic gunslinger (or swordbearer) glorification - the hero of the frontier, with a dose of period-appropriate white supremacy on top. In other words, scratch the surface and we are basically the same.
I think it's a basic dichotomy of RPGs: do you want to produce the equivalent of a Hollywood action movie, or the equivalent of a drama ? Most people will lean towards action movie: it's easier to grasp, easier to slip into and out of, provides immediate satisfaction, and you maintain a healthy distance from your slightly bloodthirsty and smilingly sociopathic character. It's the same for video games: action games outnumber (and outsell) narrative games by at least one order of magnitude (and probably several). It's not a difference between US and Europe so much as a difference between emotionally simple and complex. Action games speak to a very primal part of us, the part who yearns for a simple world where violence is a legitimate answer to every problem (and where you are proficient in dealing it).
It's only more obvious in the US because the gun culture is out in the open (the US history of being built by the gun is still recent, ours - built by the sword - a bit less so), but I wouldn't trust many of my fellow Europeans if anyone had access to a gun here.