One thing I don't understand... why is it always slaves?
Why can't it just be someone they befriended in the "another world"?
ʸᵉˢ ᶦ ᵏⁿᵒʷ ᵗʰᵉʳᵉ ᵃʳᵉ ᶦˢᵉᵏᵃᶦ ᵃⁿᶦᵐᵉ ʷʰᵉʳᵉ ᶦᵗ'ˢ ˢᵒᵐᵉᵒⁿᵉ ᵗʰᵉʸ ᵇᵉᶠʳᶦᵉⁿᵈ ᵗʰᵃⁿ ᵉⁿˢˡᵃᵛᵉ ᵇᵘᵗ ᵗʰᶦˢ ᶦˢ ᶦⁿ ʳᵉˡᵃᵗᶦᵒⁿ ᵗᵒ ᵗʰᵉ ʳᵉᵈᵈᶦᵗ "ᵐᵉᵐᵉ"^.^ ˡᶦᵏᵉ ʷʰʸ ᵈᵒ ᵗʰᵉˢᵉ ᵖᵉᵒᵖˡᵉ ʷᵃⁿᵗ ᵗᵒ ᵉⁿˢˡᵃᵛᵉ ᵒᵗʰᵉʳˢ ˢᵒ ᵐᵘᶜʰ ᵃˢ ᵒᵖᵖᵒˢᵉᵈ ᵗᵒ ʲᵘˢᵗ ᵍᵉᵗᵗᶦⁿᵍ ᵃ ᶠʳᶦᵉⁿᵈ^?^
Control. It all boils back to the need to control a living being; whether the weeb in question genuinely sees those they're controlling as a human or not.
"I woke up in bideo bame world" is a shortcut to "everyone here is 'NPCs' and therefore it's okay to exploit them." 
This has given me an idea for at the very least a short story that plays out like your average Kyle video game isekai power fantasy for the first little bit only for the example to try and pull the kind of shit he'd pull in Skyrim and gets his gut turned into a crossbow's bolt quiver for it because even if the universe's logic works like a video game, these are still people and you just tried to rob a borderland merchant
I do not want to see a TRON remake where the "user" shows up on the Grid and their immediate response to that is 
because bideo bame.
I think a controversial one that irked me was Nidome no Jinsei where people claimed dude was a soldier during rape of Nanking. I had to do a double take on that shit
It would take more effort and writing and creative justification to make a friend that may become a romantic partner in the fantasy world.
It's more efficient to churn out waifu slave harem slop because the treat hogs will still gobble it up. 
It’s so bizarre that despite me being a card carrying weeb, the only times I’ve seen Isekai done sort of well is in western animation: The Owl House and Futurama (for anyone looking for a more sci-fi flavor of the genre) come to mind.
There are some Ghibli films, but I’m talking more about animated series here.
Now I want to make an alignment chart about Isekai.
There’s a scale between media purist and premise purist.
So something like I reincarnated as a slime can be Isekai, or the fresh prince of bel-air is isekai.
Most shonen is a power fantasy. Isekai is just an excuse to drop every pretense that it's not.
True but at least many of the shonen protagonist don't abuse their power like isekai do and have some moral code. Although several eat enough food to solve global hunger many times over. 
Why is it so rare for a new arrival to a fantasy world to not be a horrid asshole to that world and ruin whatever made it special in the first place? Like, "behold, a land not yet devastated by capitalism, time to bring capitalism to it!"
TBH, it could be a great opportunity to dissect myths about early capitalism and show all the genocidal shit capitalists did for their capital.
It might be hard to show that and not have weebs and other chuds go "based" about what the world destroyer is doing to destroy the fantasy world, unless it was delivered as harshly and uncompromisingly and without Gambo-like gratuitous portrayal of anything that hogs might get off to anyway. Basically an isekai version of "Come and See."
12 Kingdoms, Rayearth, Escaflowne, and El Hazard were decent Isekai from what I remember. They're all from the 90s/00s though.
They still had trash Isekai in the 90s, like Those Who Hunt Elves. 
I heard there was an isekai where the mc kills other isekai mc for abusing their power. Idk what its called
I think this may be it. Idk it sounded like some task force going universe to universe to murk abusive ones. Lemme check
IIRC the person who wrote the story actually made the lower part of this meme as well as a teaser for the story
I found a YouTube link in your comment. Here are links to the same video on alternative frontends that protect your privacy:
Who would win:
Some mediocre creepy weeb that fell unconscious for whatever reason and woke up in a fantasy world that he is ruining with video game exploiting strategies?
Or one old guy with a loaded Beecher's Bible? 