Earlier quoted context omitted.
Its about showing off to other players. You can still do local game mods just fine but that's not what people are after.
Which is an easy technical problem to solve, but the liability of abuse when sharing user content with other users is not palatable. It is also not impressive to others, not a status symbol, and that's actually the purpose of skins in the modern day. No one grinds 1000hrs of warframe for a skin just because they think it looks cool, they think it makes THEM look cool. They want people to be impressed that they had $2…
Looking back to ~2012/2013 and its seeming to be clear now that the introduction of weapon crates, the steam marketplace, and all of the other MTX in all of their(proprietary) competitve games may have been a good indication that these would be the last games Valve would develop in-house.
To be fair though and just to give a counter-example, the "clout chasers" with the $1000 knife skins is essentially the same as the bragging rights of a 4/5/6 digit steamID during 1.6 and CS:Source. Although flexing SteamID length was something I only really saw in the competitive scene and of course had a much smaller(unofficial) market.
Oh well, RIP Steam games, long live Steam software(their platform/Proton, etc) and hardware...minus the steam controller.