I agree: isn't the whole point of the adventuring party that they are all exceptional? Drizz't was the only good Drow, etc.
To use your example, Drizz't was the odd person out in the party that formed and even he had a grounded reason why he was in the region and a connection to it. Another example might be the Heroes of the Lance from Dragonlance which was a diverse party as well with very exceptional individuals that all had solid ties to the setting and region in which they lived.
The "circus troupe" language makes it seem like your problem is the weird new races, not the lack of regard for your worldbuilding or your desire for immersion. Both of which you're doing for everyone's benefit.
Part of the problem, I think, is that you're treating humans and "old school" (and more humanlike) ancestries as automatically normal and justified and newer and less humanlike as being automatically out of place. In a way, you're doing the same thing to yourself.
On the one hand I feel like you get where I'm coming from and then you latch onto this concept that I feel like I have dispelled multiple times in this thread.
I do not have any problem with weird races. Players can be gelatinous blobs or flying monkeys and we can have a great game. I do not have any enormous attachment to the traditional ancestries. What I am against is a group that does not fit the demographic makeup of the region the campaign is set in UNLESS that is part of the premise of the game.
I have no issue with someone playing outside that box but it shouldn't be everybody. I shouldn't have to write in a random tribe of rock trolls that live "across the western mountains and a little south of the Old Trade Road," just so I player can pull out their Rock Troll Wizard that they created three months ago.
I feel that too many players show up and dictate their characters to their DMs and just expect the DMs to deal with it without considering that maybe, just maybe D&D is a collaborative experience and just slapping down your disconnected character lowers the experience for everyone.
The thing is, not all players are interested in settings, backstories, lore and roleplay. Some just want to have a laugh and kill some monsters. A player who enjoys that sort of thing will be able to make the most weird character choices fascinating additions to the world. But a disinterested player can make a human fighter feel like an anachronism.
That's totally fine, but then those players shouldn't join custom, roleplaying heavy, campaigns. Perhaps they could join Adventurer's League, DM a beer and pretzels game of their own, or break a game of Munchkin?
part of the problem is no one can see settings before they show up.
assuming that what both sides of the table can even find common ground on desired settings.
was this a reason for pre made setting to exist?
I think you are certainly on to something there about pre-made settings.
Those characters absolutely fit into a Strahd campaign! Honestly what's it matter if they have horns or tails or a shell or if they look human? It's the character motivations that matter.
Exactly, it doesn't. You said it perfectly, those characters fit the campaign and are tied to it through their motivations and backstories. Additionally, Ravenloft is a bit of a separate case since it is a setting where the expectation is that the entire party will be outsiders.
That's just the old "Can I play a ninja?" trope. Some players just want their characters to stick out in the setting.
That's totally fine too, but making the entire party like that, having everyone disconnected and apart, every time is exhausting from someone like myself that enjoys depth in story and roleplay.*
*That's not to say that excellent story can't exist within those confines (or lack thereof) but rather that it is much harder to bring them out.