Why do so many Isekai stories have child abuse disguised as "training"? It is not how real effective training of anyone child or not works. Some training programs use high stress training to weed out people during the training period. But it is not part of skills training. It is used to test their resolve and ability to work under stress.
Forcefully making kids skip meals when they fail. Is another abuse example. That hurts their growth more than just getting beat up and stabbed.
Look chief, we look down upon child abuse as well as anyone else, or that I HOPE you do.
Now that we've established that child beatings bad, I can now answer your question of why isekai stories, or stories in general disguise child abuse as training. Simple answer is because it's been done before to GREAT effects, and the abuse you're mentioning are pale imitations of 'em.
But if you want the elaborated answer, here you go:
I'm sure Berserk needs no introduction. If you've read through the manga from start to """finish""", you can see that Le Black Swordsman didn't have a good childhood. But it was through that hardship of a childhood that you get the mega tank you see in the Golden Age arc. Now, I'm not saying you have to throw a kid into a pit of wolves to make him a giga swordsman, but for what the story is trying to tell and the tone it sets, that is what happened and that is the result of it. By no means does Berserk try to conflate his hardship with his glorious skill. Hell, glory barely comes for dear Guts throughout the manga. It's just a nonconsensual means that's led to this end, and has always been treated like this.
I'm not saying Berserk is the key influence to this trope, but I'm bringing Berserk up as an example. Notable mangas or comics from the past have used the "punished child becomes the strong MC" as a way to establish their characters. You got the fatherless Batman, the cursed destiny of Hellboy, and so on and so forth. The pain defines the character, not just their strength but their character as a whole. But as always, there will be poor imitators looking to get a taste of the pie without working for the slice.
Hence, you get these tropes, where the children are thrown into absolute hell by their parents/guardians and we're there to watch as they crawl back up a bigger man than before. This is the easiest cop-out you could ever make. Right from the get-go, you have a pre-established past signified by the parents, an easy, glanced-over backstory of "child goes through Dante's Inferno's worth of hardship", and an automatic absolution in the question for the MC's strength because, well, if they can survive Satan's trials as a kid, they can definitely survive the real world. It's a free pre-established character wrapped in a little neat tie, like an author's favourite ready-to-eat meal.
And as for whether it's realistic or not, lmao ofc not. You and I have seen so many arguments and discussions about swords vs spears and magic elements and stances and what not to easily end on the conclusion that realism is ultimately a situational dressing for your story depending on what kinda tale you're putting out. If we were to apply your idea that child abuse training is unrealistic to a story like Baki, then Baki would just fucken suck, since the whole story runs on insanity as a baseline for its throughline. That concept works more on grounded stories, where the realism adds a needed flavour to make it THAT more memorable for readers with an eye for detail.
TL;DR: People do "child abuse training arcs" because it's an easy cop-out for a strong character. No, it's not realistic, but it doesn't mean it's automatically bad. The trope is low-tier, yes, but when used situationally, it can be a good starting point for a story.