First, I'm not really a diehard LitRPG or even from a broader scope, GameLit fan. Second, neither are really "new" per se. They've been around for a few years now. If you throw in stuff from Japan, they've been around for a few decades.
Since I'm not really a diehard fan and can't argue from the perspective of one nor one who frequents their communities religiously and engages in their conversations about why this became it's own separate genre rather than just fall under stuff like sci-fi, fantasy, etc., I'd say that it became a genre likely because those genre fans got annoyed and gatekept their genres, which is fine, when LitRPG stuffs showed up. I've seen it happen elsewhere, so I think that's a safe assumption to be made here.
Most LitRPG books I've picked up, in my opinion, can't really be likened to Choose Your Own Adventure books as they're written around a particular character and set of author determined plot points. Can you argue that a reader can self-insert into them? Sure. But you could argue the same thing about nearly any piece of literature. Even some not particularly written for that purpose.
While it's obvious some authors have never played a video game before in their lives or only picked one up to write their book and try to capitalize on the popularity of the niche, I'd argue you're painting with a broad brush claiming "authors" are "copying what they believe games to be like despite not playing anything in the genre and taking those stats as hard facts of the original way they were used..." I can say that I know (as an author in other genres, myself [who may or may not get into LitRPG]) that sometimes these niche genres have to write stuff based on what the fanbase wants. However, sure, some are like Table Top RPGs, because that's the point and in some cases making the stories too much like TTRPGs hurts the narrative and its verisimilitude. But, if you go through some of the reviews on Amazon, Audible, or where ever, you'll find a ton of people loving that kind of thing. Which, goes back to my point that some of these niche genres or subgenres incorporate what's most popular among the fanbase.
Also, to your point 3. As a gamer, there are a significant amount of games where the stats do incorporate a sort of one-for-one usage in abilities, etc. depending on how developers define the stat.
To be fair, the genre/subgenre (whatever you want to consider it) isn't without it's problems, in my opinion, but these are personal pet peeves. 1) I think too many authors in the genre don't fully know how to plot out a story properly because the vast majority of LitRPGs start on Royal Road or take on a slice-of-life plot device or what I like to call situational slice-of-life plot device where there's no real end goal in sight. Yes, even if the story is full of conflict, action, and adventure. 2) Too many become wish fulfillment power fantasies where the MC's inevitably become over powered and so, reading about their adventures become boring because there's no challenge to the MC. 3) Something that I can agree with you if I'm fully understanding your final point, but too many authors build a system and then ignore it. Have levels and then ignore them. For example, from a gaming perspective you wouldn't have a level 5 somehow magically defeating a level 35 monster without some sort of special cheat. And, yes, too many authors will invoke cheat skills to allow their level 5 MC to defeat monsters and villains several levels higher than them. Etc. Etc.
I'm sure some diehard LitRPG fans may come in the chat and do a better job at defense than I did. Not that I was really defending, I'm just bored on my day job and I was looking for something to do and saw this topic and it interested me. I've put down more LitRPG series than I finished and so far, have only been sticking with Dungeon Crawler Carl and to date, that's starting to wear thin on me.
