Got drunk, installed a hentai game, fell asleep with it running. Next day it looks like I have 7 hours spent and I can't remove it from my account. Thanks steam!
If Valve weren't hellbent on trying to leverage social networks to sell games, the default for everything would be private. How many people want to notify everyone they know about their game collection, how many hours they've spent on each, whether they have their computer on and Steam client logged in at any given point in time, or what they're playing at the moment? It's not as if one's grocery store, bank, library, car dealership, stockbroker, electrical utility, video streaming service, or any other vendor has a mechanism to broadcast purchases or useage statistics, much less defaults to having it enabled.
There is a legit need for matchmaking with friends in multiplayer games, sure, but that'd be readily doable as an opt-in on a per-game basis and visible to people who have the same game.
And even for that case, I'd also wager that there are people who would prefer to have different profiles for different friends groups. Like, maybe you want to have a different public face when hanging out with a handful of real-life friends, your spouse, your six-year-old kid, and your guildmates in some video game, even if you might, at some point, want to play one game or another with any one of them.
You can run games in Proton without Steam unless that game actually depends on Steam integration. If you just want the Proton compatibility support or whatever.
That being said, Steam Input can be a useful layer to configure controllers if you use a controller, and that's tied to Steam.
EDIT: Also, that's a generic approach for any Steam games.
Looking online, specifically for RPGM games, it looks like they're internally web-based. You just need a local webserver. I keep a local-only-accessible instance of Apache for running local web-based apps, but it looks like there are people here using the Python mini webserver for something that'll be more-trivial to set up:
Might be able to open a file directly in the browser too -- I see some people there talking about that -- but that'd let any scripts on the webpage see other files on your machine, so probably just as well to keep it sandboxed to what the webserver can serve. Probably not too much of a concern if this is a game commercially sold on Steam, but a good practice more-generally. I kind of suspect that this may be why Android web browsers don't allow viewing local HTML pages via file:// URLs.
EDIT2: One caveat is that I assume that RPGM probably uses localStorage for saved games, and browsers default to limiting how much a given domain can consume, and if they store a bunch of data, and you play a number of games, it'll probably burn through the cap for 127.0.0.1. Twine games serialize a bunch of JavaScript objects, and definitely can exhaust that. It doesn't look like you can set different storage caps on a per-domain basis for Firefox, though you can increase the global per-domain limit:
Ohh its to protect my kid from whatever twisted porn game I find.
There are only two people on my steam friends list, one of them is my kid I buy games for, and the other is my wife. My kid doesnt care that I'm playing BG3 or Satisfactory for the 1,000th time, and my wife's reaction to finding I was playing a porn game would probably be "You know, we could just do that for real".