It's about consistency - you want to build an app that looks and functions the same on all platforms as much as possible. Regardless of if you are hand-coding or vibe-coding 3 entirely separate software stacks, getting everything consistent is going to be a challenge and subtle inconsistencies will sneak in.
It comes back to fundamental programming guidelines like DRY (Don't Repeat Yourself) - if you have three separate implementations in different languages for everything, changes will be come harder and you will move slower. These golden guidelines still stand in a vibe-code world.