As most players have the bad habit of casting everything on their first main phase, it's incredibly likely they'll move to combat and realise they've splurged all their mana and can no longer afford to attack you
Its not colorless and the colors of mana in its mana cost
The oracle text really helps here since it reads At the beginning of the end step, destroy all untapped creatures that didnt attack this turn, except for creatures that couldnt attack. If we look back at the Arbiter we see that there's no restriction on which creature can attack, so summoning sickness aside on my turn, it is likely that all but one of my creatures will get destroyed
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Rather than be the focal point, Szarekh supports the deck by accruing card advantage and filling up the graveyard and he's less likely to be a target because of this
