Because ones bike is a carefully selected item perfectly fitted to ones body shape and preferences, whereas the battery is simply a line replaceable unit.
Privileged enough to fly somewhere specifically to ride bicycles? C'mon, I think you can find a good enough bike to rent when you get there.
I've also done it the other way, my main bike has S&S coupling so I could bring it aboard the Eurostar. For touring I prefer my own bike, because I have the racks set up for my panniers, but when I do that, I prefer travelling by ferry/train.
You totally have the option to fly with a regular muscular powered bike.