Because we shouldn’t expect cartoon characters to at all act in logical ways or for consistencies to pop up & logically expect them to act certain ways.
& just because you don’t necessarily ship something with canon evidence doesn’t mean you don’t have shipping goggles instead of eyes.In fact, your extreme insistence that Rarity is attracted to Twilight just because she squeezed Twilight’s face & calls all her friends “fabulous” far more strongly suggests your eyes being shipping goggles.