Because unless there's some weird hidden meaning to the kiss scene in 68 that we're missing, or if it's a red herring for some reason, there's clear reason to suggest that Itsuki had become fully and unambiguously aware of her having romantic feelings for Fuutarou that were strong enough to prompt her to do that. The kisser had to have strong enough feelings to kiss him at that point. Nothing really suggests Itsuki did unless you really want to stretch things and then rely on "hiding emotions" or some bullshit. Yotsuba hides her emotions, too, but everyone reading it more or less knows how she feels about Fuutarou.
We know she was friendly with him, but everything beyond that suggests that if she had romantic feelings for him she either wasn't aware of it, was in denial, or still wrestling with them/too uncomfortable with them to make a move.