Consider that in 1st edition, only humans and half-elves could be player character druids, and only humans, half-elves, and half-orcs could be player character clerics. The game was still enforcing a very human-centric perspective in 1st edition, and having adventuring priests was very much a "human" thing, likely as a way of channeling some of the literature that D&D was based on (e.g. Poul Anderson and the whole humanity = Law, faerie = Chaos thing). Some of this (such as Druidry being a Celtic-inspired, specifically human belief system that only humans and half-elves engaged in, not "generic nature-worship"; and paladins being human-only warriors for Good and Law) held over into the 2nd edition. There was no reason to allow elves to be druids or dwarves to be paladins, because elves and dwarves weren't supposed to be human enough to belong to those classes.
Yes, and it's still nonsensical. As I said, it was a profound flaw in 1e and 2e to put wholly arbitrary restrictions on classes like that. It was hard-coding
very specific setting presumptions into the game, far more specific presumptions about the game setting than we have seen in later editions.
So, there are only a handful of character classes in the world, and only humans can be all of them, many of them are human-only, and only humans can go to 20th level or above (or half-elves can be unlimited as bards, and in 1e races had ONE class they could have unlimited progression in).
Yeah, uh huh. It was
always a profound design flaw in AD&D. It's one reason that every AD&D game I played in used the optional "exceeding level limits" rules in the 2e DMG to raise the level limits, and almost every game I knew waived or at least relaxed the racial restrictions on classes. Humans couldn't multi-class, but they could dual-class. . .but only humans could dual-class, and even then only if they had extraordinarily high ability scores. It was just a total mess of inconsistent rules.
When I was getting involved with D&D in the 1990's, every group I knew had a collection of their own house rules in how they changed the game from RAW to rewrite things like that, because even at the time D&D was seen as archaic and outdated, and those setting-presuming rules were part of it.
The idea that druids were more-or-less just generic nature priests and not very specifically locked to human Celtic tradition, and that Paladins are general Lawful Good holy warriors instead of a human-specific class was pervasive in the D&D fanbase (at least from my experience) by the 1990's.
Tossing out level limits and arbitrary and nonsensical racial restrictions on classes was one of the many reasons pretty much every D&D player I knew rushed to adopt 3e when it came out, because 3e dispensed with those rules and felt like it was written to reflect the game we were playing (or wanted to play) and not Gary Gygax's humanocentric setting worldview from the mid-to-late 1970's that was a mix of Lieber, Anderson, Vance and Tolkien.