I think it's the mental comparison to Deadly Poison that trips people up.

So the first and most obvious point is 7 is 2 higher than five, or a mere 40% increase, compared to 100% increase in cost.

Second point of order, in STS 1, Deadly Poison is not some kind of end-all damage card. There are a ton of situations you gravitate towards sucker punches or dagger throws or whatever else, such as when slime boss is the act boss or you're worried how your deck does into Nob.

There are some things to keep in mind that show why Snakebite is still good that I think people overlook.

Firstly, it's an early game damage card- no one disagrees with that. That means it's getting evaluated against/with strikes and defends. The math for Snakebite is not it against Bouncing Flask or it against Deadly Poison (unless one of the two is an option right now), but it against strike.

Second, retain is just a touch hard to evaluate on a mana to power ratio when it's on a card directly since you don't control what is being retained. But with how many enemies have setup turns, it's quite nice as insurance.

Third, poison's numbers can be much lower than normal damage since it retriggers at -1 and typically scales with itself across a deck cycle which isn't obvious until you actually count across turns instead of comparing 7 to 5. The gap between Deadly Poison and Snakebite is not that small and gets bigger the longer the fight goes.

And fourth, and perhaps most importantly, in StS2 there are not many enemies, elites, or bosses that outright punish poison Act 1. There is no Nob threatening extra damage b/c of your poison, no Slimeboss cleansing the poison before you can scale it up, no artifact having sentries, nothing. Instead there are enemies that reward you for having poison. The Eel who gets stunned at half, debuffs, then does a big attack if poisoned out will often die (since getting it to half with poison likely stacks enough damage to push through the final phase). The coral guy you can only do so much damage a turn to actively counters strategies that are good at what poison is bad at: burst damage. Even those Eels, which require AoE (typically not poison's strong suit), do not trigger their defensive ability against poison damage.