BECAUSE PIRATES HAVE BEARDS, THAT'S WHY.
I'd previously mentioned that showing Susan representing a vampire card would be a bit weird, but here she is as a werewolf! Sort of. It's possible I just wanted to draw Susan with a tail.
The exact mechanics of what's happening in this comic isn't really all that important. What matters is Sarah used STRATEGERY! Er, strategy. Nonetheless, I hope it's clear that, if she HAD tried summoning Aludrakrala, it wouldn't have worked, AND she wouldn't be able to simply try summoning it again on her next turn. She would have lost her chance to use the card for at least this game.
Incidentally, I've totally done this sort of thing before, complete with pretending to be upset that they took the bait and inner maniacal laughter.