I do wish that isekai characters would wear different clothes every now and then (I'd love to see a cattlepunk or afro-futurist isekai.)
I don't have anything new to add wrt why people do worldbuilding that won't feature in their stories, but I will say I also have some trouble understanding people who worldbuild 'minutia', as opposed to someone like me who does a lot of 'unnecessary' worldbuilding but builds 'outwards', integrating new things whenever cool ideas occur to me but not worrying too much about 'filling the gaps'.
My first assumption is it's born of 'anxiety for not being perceived as "legit" enough' as @Kuma put it, or not wanting to be nitpicked by CinemaSins-style pedants (although I'm 99.9% Sure CinemaSins is only being nitpicky for comedic/entertainment purposes and doesn't actually seriously believe most of the 'sins' need to be addressed). But maybe people genuinely enjoy being thorough like that, and maybe it's rude of me to assume it's fear/obligation just because I personally find it tedious ...
You need a banner :]
Exceptional meme, @NickRowler. Exceptional.
I think you have to strike a good balance between a relationship driven narrative and a world that feels lived in. You can't dawdle on details, but if there's no there there in terms of a setting (especially for fantasy/sci-fi) I tend to find it falls flat.
To me, success is walking that line and creating a compelling mix of both. And it's hard to do if you have a preference or flair for one or the other. It just takes practice, and some folks are earlier in that process than others! 
I suppose is some kind of Futuristic Western setting.
@Stargazer31 I saw a Sword and Sandals Isekai for a change, that looks more Greek or even older, instead of generic medieval.
Bruh, just say you don't know how to make your world interesting outside of making reading material. It's okay, we won't tell anyone, promise
this is a joke, do not come after me pls i don't do well with confrontations
some of us just like figuring out little things, it makes the world come alive! I don't think it's garbage to enjoy worldbuilding, and while I'm definitely not extensive with it, I think it's cool when people know all the little things about their world because they love it so much.
While yes, you can't include every detail, I don't think it's stupid in the slightest. It really is about how you present your information to the audience.
I can explain people who world build too much!
Take JK Rowling, for example. Do you know how wizards relieve themselves of their bodily wastes? I do! Thanks, Rowling!
EDIT - I probably don't want to bring up the subject of JK Rowling right now because she just gets worse every time you hear from here
Yknow people buy Dnd books all the time, people have jobs about the lore of Tolkien books, I'll bet everyone here had a dragonology or wizardology or whatever book as a kid, the angelarium and all future tomorrow's and warhammer and scp archives are all just lore and people love that shit! It gives them a new world to explore the depths of and a new perspective on reality through the lense of an authors (usually multiple authors) imagination. I get that when you're reading through a fiction on ao3 that seems like a standard love triangle you can be blindsided by the info dump of the last 13 wars and revolutions of a minor island off the peninsula of who-what-where-now? But lore itself is fun, in its own way and can be used in lots of interesting ways to tell stories that you can't with just a few characters, like alot of people have said previously in this thread its not what you write its how you write it.
I tried to work with writers for some years and I read a lot of the
giant world - no story / story which is not understandable scripts.
As an artist I enjoy a very detailed character description and this is something which is, in my opinion,
very important. I like to know every little detail about the character and that´s also something which
is interesting to read for me. From their parents/where they were born over how they grew up to
what´s in their fridge, which medicine they take, what they read etc
And again, this is just my perspective as an artist reading scripts: there are many scripts with
crazy big, detailed worlds with characters you can´t count with nothing interesting happening in
the world. I love scripts with 1-3 main characters which focus on the characters and not on the world(s)
around them. What do they want, how do they get it, what gets in their way etc the normal storybuilding
blocks and not 50 characters in universes that I don´t understand, how is the reader of the comic or novel
gonna understand that?
Well, you were doing fine until you wrote this.
EDIT: You know what? Never mind, it's too late. Sigh.
To go back to the main question : Can someone explain people who “world build too much”?
One very down to earth reason for me is that I like to keep my mind busy thinking about my world/characters.
Since it takes much longer to write my stories than to create them in my head I keep adding stuff to my world every day.
Some people like that kind of story.
A friend of mine loves the movie Dune, citing its world building in particular, while I was bored by it being more of a character guy.
Ideally a story should have a balance of good world building, plot and characters,
but I find that I can forgive a dull plot or shallow world if the characters are engaging.
So I think there's an audience for a world building focused story, even if it's not you or me.
Dune is a good example what´s so difficult about building a world like that.
The 1970s try to film it was such a failure, they made a documentary about it.
It´s called "A documentary about the greatest movie never made"
The 1984 version was a flop even though they had all ingredients, famous actors,
director, design etc. 30 million$ budget.
The 2021 version was successful, I have no clue why 
3 trys to film a movie.
For me this is a good argument not to create too complicated big worlds,
especially when you are in a webcomic, entertaining online novel genre
