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Because that judging guideline only solves if your opponent plays a card during your action. How long do you get to decide of you are playing a Dom after your action? Half a second? A second?
All the while your oppenent is waiting, so he plays his Dom and suddenly no no you were just about to play one responding to own action! Responding to your own action is yet another bandai. Its helps judging but doesn't actually solve the problem.
That would be fine with Dominants followed initiative but they don't. Initiative is a clearly defined mechanic for when you can play a card, that Dominants completely ignore.
Asking your opponent "would you like to play a dominant" is a verbal que to them that if they don't, you are likely going to, it gives them an advantage, and all because we don't have a clearly defined rule for "dominant initiative"