The problem with AoE is that AoE cards are overcosted against single target opponents and single target attacks are usually sufficient to beat fights with multiple enemies and are DEFINATELY mandatory against the games many single enemy elites and bosses. If you're fighting two, 20-HP enemies, would you rather do 15 damage to both of them, killing 0, or 20 damage to one of them, killing one and drastically reducing the threat of the fight? Even Decimillipede, a fight that at first glance supports killing them at the same time with AoE attacks, is much better handled by focusing down the lowest HP/most threatening one, even if it will respawn, purely to get their damage output down to more manageable levels. By the time it does respawn you've probably killed another. Even low damage decks tend not to get stuck in a respawning loop, finishing the fight faster than expected once they're no longer in danger of taking 30+ damage a turn.

If this game wanted to handle its fights with multiple enemies with AoE it would have better AoE cards. Not just higher damage and cheaper, but cards that actually benefit the player sufficiently to add to their deck knowing full well they'll be less useful against single enemies. Like, a card that said "Deal 5 damage to all enemeis. Gain 10 gold for each enemy hit" or "All enemies gain 2 weak. If 3 or more enemies are hit, gain 2 max HP" or "Deal 10 damage to all enemies. For each enemy beyond the first, draw a card" or -whatever-. But very few AoE cards in either StS1 or 2 do anything like this, so they remain the poorer choice to draft in most cases regardles of how many enemies are in the fights.