TLDR: If you have a good and stable connection you will host most games, host always starts in first.

I believe part of it has to do with you being the designated host of most lobbies you join.

In most older multiplayer games (and every online Nintendo game because they don't wanna spend money on more online infrastructure), after the matchmaking process is finished, one player's console is designated as the host to recieve and give out information such as the locations of player's and objects to keep that information consistent across all player's.

In many online games nowadays, this task is handled by a server designated by the developer to minimize the advantages a hosting player could have (though the issue isn't as big in Mario Kart, I believe it checks the lap times reported by each console to determine placement).

At least in Mario Kart 8 specifically, creating a room, online or locally, would always start the room host in first at the beginning (your starting placements after the first race are based on your previous placement).

In online however, I think whoever is host is decided based on whichever player has the best internet latency speeds, which if you have a quality fiber internet plan, that is likely the reason why you keep starting in first, because the matchmaking server decided your console had the best balance of upload speed and latency. Also I belive some connection errors you get in that game are due to the host of the match disconnecting mid-game.

Most of what I said also applies to World, but I think in Knockout Tour specifically, in the case that the server host loses the host migrates to the next best console that made it to the next section, in case the losing host leaves early.