Ok this makes sense to me... but it also shows how artificial the "old school" is.
Not at all. There is huge variation with the OSR. After all, it's been around for 15 years, and become quite popular. And it's always been a grass roots movement. Anyone can claim anything is OSR. But there are core philosophies as well.
One of the original principles of the OSR was an attempt to rediscover the original playstyle. And by original, I mean original. I'm not talking about the broader West or East Coast scenes in the 1970s. I'm talking about two(three?) cities, Lake Geneva and the Twin Cities. And more specifically, two tables, and two campaigns: Greyhawk and Blackmoor. Or just Greyhawk. That's the style promoted in things like Matt Finch's Primer. In the early years, the OSR was very heavily focused on that very specific playstyle. It was about figuring out how the rules were supposed to be used, by the people who created them, instead of the many diverse ways they ended up being used, by all the people who picked up the game later. It was also very specifically focused on the three little brown booklets of OD&D, and maybe (maybe) the supplements, though some even considered Supplement I: Greyhawk (the very first supplement published for any any RPG) to be a step too far. (To be fair, it did radically change the game.)
There was also a more general focus on the wild and woolly days of the 1970s. Not just Gygax and Greyhawk, but things like Arduin, Alarums and Excursions, and all the Judges Guild products. It was admiration for that huge surge of creativity and wild experimentation, but not an an attempt to copy anything in specific. Instead, they wanted to recreate that flowering of ideas. This led to things like Fight On!, Santicore, all the weird little zines, and even the blogosphere's obsession with random tables. It very much promoted the idea of self-publishing, genre mixing, sword & sorcery and weird fiction influences, and more.
There were other influences as well. Despite it's obvious connection to the first and most influential clone, OSRIC, AD&D 1st edition has always been weirdly marginalized. A lot of that has to do with the communities. The original OSR crowd were people who were dissatisfied D&D third edition, tried Castles & Crusades, and then had a falling out and created OSRIC. They were (mostly) distinct from the people who never stopped playing AD&D1e. The latter group were significant, but underrepresented on the web. One of their few hangouts was Dragonsfoot. So there was more focus on OD&D than AD&D.
B/X also became very popular, though that was a newer trend (not too new; this was still the mid to late aughts). OD&D is purer and more wide open, but B/X is tighter and more concise, and an easier entry point. For those who were less interested in capturing the original playstyle but still liked old school games, it became the default. Labyrinth Lord prospered, and newer games that weren't really clones but were old school inspired, like Lamentations of the Flame Princess, most commonly used it as a base. It kind of became the Rosetta Stone of systems.
There's also a persistent reverence for the original players. Not just Gygax and Arneson, but also people like Mike Mornard, Robert Kunst, and Tim Kask. This is strongly associated with the attempt to recreate the Greyhawk/Gygaxian playstyle, and while it exists, its importance is typically overstated by people critical of the OSR. If was a driving force, then Robert Kunst would have made more money than James Raggi, but that's clearly not the case.
Since then, the OSR has gone in many, many other directions. It's become very hard to define. But understanding a bit about its origins and where it came from, and the major trends, can help get a handle on it.