So take out the strategic decision-making aspect of game theory and you no longer have a game. Yes you can take the abstract mathematical logic of game theory an apply it to other areas (computer science), but my simple point is that the initial impetus behind developing game theory was to answer questions about decision-making under constraints, in this case the constraint being the actions of the other players of the game. Remove the players and you don't have a game to theorize about.

There are many fields of study that are formalized into mathematical language. That doesn't make them mathematics.