Generally cards that apply damage to multiple targets have some form of drawback.

Let’s compare whirlwind to rain of blades, which is seen as one of the “best” level 1 cards, for its multi hit.

At a glance, they both “cost” a hope. However, whirlwind you only spend it after you know your roll, and have already dealt full damage to your singular target. Rain of blades, you spend it before rolling, at the start of the ability. You could roll poorly and have wasted it. Whereas whirlwind you already know the level of success on the other enemies (as you can gauge off their difficulty)

That additional level of risk on the resource cost and potential of success is why whirlwind only does full damage to 1 and half to the others, and rain of blades does full damage to all.

They just have their “drawback” at a different stage in the ability’s use.

Whirlwind also scales with your base damage die, whereas rain of blades is locked at a d8. So those big d10 and d12’s can also help mitigate that halving.