You know how in Kanto you can just click enter to shift between the islands on the map. It'd be much easier if we could just click enter and it shifts between Kanto and Hoenn cause (especially for if you're making a trade) it takes time to go through the slateport - vermillion thing
There is a line of balance between immersion and convenience, and a command would cross it - that would be some really crummy design.
It would also be more difficult to understand for those unfamiliar with the game.