>>96333960
1. Because the game is fundamentally an interactive back-and-forth. Mechanics above interaction only work if the entire game was designed around them from the start, which MtG wasn't. Even then, a game that WAS designed around a mechanic being uninteractable is prone to breaking if it improves too much (Hearthstone hero powers with Baku and Genn).
2. Because emblems are no longer a big, flashy payoff for getting a planeswalker to ult. There is currently a 3-mana planeswalker with a stacking emblem on their +loyalty. Emblems are becoming easier to access and more powerful as time goes on which makes it unhealthy that they're still being given "can not be interacted with" design space.