Because theres a billion of them, all beckoning you
It is much less reliable for final rules MTGPress proxy printing became popular because it solved a very specific Magic problem: players wanted a fast way to turn a decklist into printable proxy TLDR Fan-made Magic cards feel real when they follow the same invisible rules that make official cards readable, balanced, and playable
It also isnt as oppressive as other Izzet spellslinger commanders, and I actually like giving away information for free if my goal is to teach a newer Magic player what blue and red do together
One card I created was the following: Fighting Spirit 1WW Creature Spirit 2/1 Flying Whenever a mystic spell is played, CARDNAME gets protection from the color of your choice until end of turn
Finally, we gave the player a secondary way to advance the Story: they could use mana to progress it
Some control decks may play a copy of Mystic Sanctuary so thats something to always consider alongside Uro or the odd Snapcaster Although we dont see these decks too often these days, for historic reference: Reanimator: Reanimation targets, Faithless Looting Hogaak: Bridge from Below, Hogaak, Arisen Necropolis, Vengevine, Bloodghast, Gravecrawler, Cabal Therapy Madness: Vengevine, Faithless Looting, Anger NicFit: Cabal Therapy however, really no specific card other than Academy Rector in NixFit builds