Well a)
you do ask them to physically turn up, so :pQuote:
In flavor terms, Chandra says, "Forget this, man—when you called on me, the deal was that I'd blow up some of your troubles as long as you protected me from the worst of it," and walks away from the fight. It's remarkably like summoning a hotheaded young fellow planeswalker, with her own agenda, strategies, and spells at her disposal, to help you in the fight against your enemies.
but b) if the designers felt the flavour of a planeswalker having their own deck when they turned up was so important that the card type had to include it, the only practical way for that to be accomplished in game terms would be an imitation of the Wishes.
It'd look like this or something.
https://i.imgur.com/IR5ueb2.png
And I'm pretty sure that would be boring and mess with tournament sideboards if they were playable and just generally suck as a game concept.