This has nothing to do with satisfying a casual audience. The reason you don't see LAN anymore, is because that requires the consumer to have access to the server code.
They could in theory do it, but it's simply a matter of saving costs, as they don't have to support an entirely different architecture from the cloud based hosting they use now. In the past, the software architecture of games was a lot less complicated than it is now.
Completely custom maps require you to have access to the engine. The Frostbite engine is proprietary and isn't freely available. That's why they, with some limitations, expose the maps in Battlefield 6 using the Godot engine, which is open source. Same goes for all other types of mods.
