Well, I got a vibe that she wants you to confess instead, and then she rejects you and says that she was just joking and she laughs at you badly and teases you until forever. I don't know why but I always think something that tragic when I face some situation like this , maybe it comes from my inferiority complex.
For everyone, saying he should confess, she's probably making him feel like she will reject him unless she beats him. It's illogical but I can personally understand the self-doubt from the MC's perspective.
@johnpoop2529 Yea, like, I get the feeling she'll probably reject him if he confessed just because of how competitive she is being here and maybe she already did once. Hence his frustration.