Remember Super Mario Maker 2? It included a mode where players could join an online room, whether with friends or strangers, to play courses among themselves. It’s also infamous for the constant slowdowns that players experienced during the courses. Why was this happening, you may wonder? Well, because the players needed to synchronize their state between each other, and since the game was not designed with modern network tools in mind such as rollback (which would probably be too heavy for the Switch), the only way to ensure everyone was on the same lane was to wait for everyone to receive the input data from all other players. And in a game with up to four players at a time, things are absolutely going to get messy.
Everything you stated has been solved in so many games in the past decade. People keep making excuses for it. Smash bros for example.
But the real reason? Nintendo just never really cared about multiplayer for Mario. Multiplayer’s not a big money maker for Mario, and they’ll implement just enough to hit whatever.
And I’m okay with that. Because I play Mario games solo or couch op.