Because no one plays the Stafford on the highest level in over-the-board tournaments (which is why there are only 7 games in total after move 4). And if you get surprised by the Stafford, other moves also work. But f3 is the simplest to learn and best according to Stockfish 14, which is why that is the response I chose
Learn everything you need to know to refute the Stafford Gambit in 5 minutes
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https://www.youtube.com/watch?v=NpVV6zroHuU