He didn’t have proof of intelligent life out there in space yet, but he was preparing to teach them how to duel. Seto Kaiba is a very forward-thinking man who plans ahead.
Honestly always thought he made the school so people knew the rules. So they dont idk use and equip spell card that raises attack to all of a suddenly use it to fusion a monster that doesnt exist and still is played on the oppents side if the field and gets weaker every turn. Then capatilize on how broken a card would be to use it to fusion your oppentes monsters but have them on your side of the board
Simple whoever had the most rizz to get the people on your side to declare you the winner just like atem did and Kiaba probably paid for. I mean look at battle kingdom before Kaiba decided the game needed rules. Mia rizzed up Joey and the gang to believing that land monsters cannot attack flying monsters yet is never brought up again